1. Introduction

In recent years, there has been growing interest in understanding the writing skills of higher education students, especially in linguistic studies. The construction of compound sentences is crucial for expressing complex ideas and linking elements in a text, but students often need support with grammar. Grammatical knowledge influences written production in real contexts. According to ^{Sudhakar and Farheen (2015)}: grammar is a system of rules that organizes words to form meaningful sentences.

Therefore, writing is a strategy that helps students develop grammar. According to ^{Sattayatham and Ratanapinyowong (2008)}: writing reinforces the grammatical structures, idioms, and vocabulary taught to students. They can expand language by writing and learning new words autonomously. It provides the effort to express ideas, and the constant use of the brain is a unique way to reinforce learning and become critical.

Previous studies show that errors in compound sentences stem from problems with conjunctions, parallelism, punctuation, and omission. Understanding these errors is crucial for educators to develop effective educational strategies. In this regard, ^{Astrini, Ratminingsih, and Utami (2020)}: studied teaching strategies for sentence structures, such as mind mapping, the PLEASE strategy, guided writing, and creative writing.

Citing ^{Sari, Syarif, and Amri (2019a)}: they described that students' writing problems are omission, malformation, and punctuation when combining two independent clauses. The author suggested that understanding these recurring errors is crucial for educators to adapt effective instructional strategies that address learners' specific needs.

The study by ^{Hendrawati (2018a)}: indicates that the most common errors among students are: malformations (57,6%), omissions (27,47%), additions (8,8%), and disorder (6,13%). Greater difficulties are highlighted in writing compound-complex sentences, especially in verb tenses and grammar. In this sense, ^{Sari, Syarif, and Amri (2019b)}: found that, although most can write compound sentences well, they face problems of omission and malformation, particularly with missing commas when joining two independent clauses.

It is also demonstrated that students have an average level when writing compound sentences. Authors like ^{Sundari, Hidayah, Edy, and Esmianti (2021)}: revealed four types of errors in sentence structure: omission, addition, malformation, and disorder. These errors were found in words, phrases, and clauses. The causes of these errors were mother tongue interference, overgeneralization of rules, and the delivery of material and methods used by the teacher in classes.

In this aspect, ^{Hendrawati (2018b)}: states that analyzing errors in compound sentences is crucial for improving students' writing competence. This gives educators key information about linguistic difficulties, allowing them to design effective strategies. In this way, teachers can create interventions to improve sentence structure and coherence, raising the quality of written communication.

Identifying common errors in compound sentence construction facilitates curriculum updates, focusing on essential grammatical strategies. This creates a better learning environment and reinforces students' linguistic skills, preparing them for academic and professional success. Interpreting ^{Erdoğan (2005)}: maintains that error analysis allows teachers to select appropriate pedagogical material to effectively teach compound sentences.

Addressing this problem improves the teaching and learning of grammar in educational institutions, as error correction is essential for success in second or foreign language learning (^{Wahyana, 2020}).

The research attempts to answer these research questions: What type of errors do students make when writing compound sentences?; What is the most frequent error made by students in compound sentences?.

Therefore, the objective is to analyze students' errors when writing compound sentences in the Fourth Semester at the Technical University of Cotopaxi during the academic period October 2022 - February 2023.

2. Theoretical Framework

3. Methodology

The methodology employed for this project was qualitative-descriptive (^{Hernández-Sampieri and Mendoza, 2018}). It focused on analyzing errors in compound sentences made by university students when writing a text. The study involved 26 students from the Technical University of Cotopaxi, Ecuador, enrolled in the fourth cycle of English language courses. The selection of participants was convenience sampling because the students were accessible and predisposed to help in the study. Data was collected from a written text.

Each student was instructed to write a short text forming five compound sentences according to a provided graphic. The writing tasks were carried out in a controlled environment. Participants were given clear instructions about the task and provided with necessary materials. Students were given 20 minutes to complete the writing tasks. Written responses were collected and anonymized for analysis.

The analysis was conducted following the five steps suggested by ^{Corder (1975)}: focusing on aspects such as sentence fragments and run-on sentences. The first step was data collection, where learners created a written text based on an image. The second step consisted of error identification, classifying them into categories of sentence fragments and run-on sentences. The third step was error description, using percentages to know the frequency of common errors. The fourth step involved error explanation, analyzing their possible causes. Finally, the fifth step was error evaluation, determining their severity and their impact on text comprehension.

4. Results

Of the 128 sentences written by students, approximately 60 contained errors. These were divided into different types, such as fragments and run-on sentences. The analysis identified cases of missing subjects and verbs, as well as the absence of both in sentence fragments. Fused sentences and comma splices were also detected in run-on sentences. Table 1 shows that the most frequent error is fused sentences at 75%, while the least common are missing subject and verb, and comma splices. The specific details of each error are explained in the following section:

Missing Subject: Based on Table 1, it was found that there are eleven missing subject errors, representing 18,33% of total errors. Students generally omit the subject in one of the independent clauses in a sentence. For example, “My father checks my homework, but never helps”. This sentence doesn't qualify as a compound sentence because it lacks a subject. The clause “but never helps” has no subject to be considered an independent clause. Therefore, the correct sentence is “My father checks my homework, but he never helps me”. This type of error occurs because students lack knowledge about compound sentence rules.

Missing Verb: According to Table 1, two sentence errors were found in the missing verb category, constituting 3,33% of total errors. Students made this error in the form of omitting the To-Be verb. For example, “Parents help their children, but this not good”. Look at the clause “but this not good”, this clause needs the To-Be verb 'is' to make the sentence correct. The correct sentence could be “Parents help their children, but this is not good”. This type of error occurs because in Spanish we don't frequently use the To-Be verb, and this could show that the cause of this error is interference error.

Missing Subject and Verb: As shown in Table 1, there was only one error made by the student in the form of subject and verb omission, representing 1,67% of total errors. An example of a compound sentence lacking both subject and verb is “My parents never helped me with my homework, so all the time low grades”. This example doesn't have a compound structure. If a sentence doesn't have a subject and a verb, it cannot be considered a sentence, as it doesn't have a complete thought. If we focus on the words “so all the time” it is not an example of a sentence because it doesn't contain a subject and a verb.

Therefore, the correct form could be “My parents never helped me with my homework, so I got low grades all the time”. To correct the sentence, we added the subject 'I' and the verb 'got'. This error occurs because students tend to fossilize the error for a long time and believe the sentence makes sense when people read it.

5. Discussion

The sentence fragment error due to missing subject is caused by mother tongue (L1) interference, as in Spanish it is not always necessary to write the subject. Students tend to translate literally between English and Spanish, which generates omissions. Similarly, ^{Puspa (2019)} observed that students need a clearer understanding of compound sentence structure, as they make errors such as subject and verb omission due to their weak knowledge of these structures.

According to ^{Rahma and Noor (2021)}: they found only one error in their study, similar to what was obtained in this study. For this reason, teachers and students must work together to improve their writing ability, especially in writing compound sentences.

The run-on sentence error is due to poor use of punctuation, which hinders comprehension. Participants do not use punctuation marks well in complex sentences, believing they are clear when they are not for readers. This agrees with ^{Ramdhianti, Kurniasih, Somba, and Akmaliah (2022)}, who found that students do not pay enough attention to commas in long sentences. Their study detected 16 errors, 35,6%. The results are similar to this study, where 75% of errors were of this type. To improve, teachers should provide regular practice and feedback on correct comma usage.

This study reveals that most students make errors by omitting elements and joining independent clauses with incorrect punctuation, due to interference from their native language. This coincides with ^{Sari, Syarif, and Amri (2019d)}: who emphasized that students have difficulties with punctuation and the correct use of connectors and grammar, affecting the proper connection of clauses. Teachers should employ strategies that improve these skills, offering clear patterns for sentence writing.

6. Conclusions

The types of errors that students made when writing compound sentences are missing subject 18,33%, missing verb 3,33%, missing subject and verb 1,67%, and comma splices 1,67%. The study revealed that the most dominant error is fused sentences at 75%. This means (45 sentences). Students forget to place a comma before the conjunction; they put it after, or they don't place a comma at all. Fused sentence errors occur due to mother tongue interference.

In the Spanish language, the comma is not necessary before the conjunction, and this interferes with the development of English writing skills. These results can be used in the design of teaching plans to contribute to the improvement of grammatical and writing skills, especially in sentence structures.

The limitations of this study lie in the number of written tasks that students did to collect the information. It was only one to analyze the errors.

However, the obtained results indicated that students do not have a clear understanding of sentence structures. It is hoped that this study can contribute to strengthening the development of pedagogical strategies as new research for teaching grammar, especially for compound sentences in the English teaching and learning process.