Language exhibits uniqueness in its ability to express various linguistic forms conveying the same meaning, while a single language form can also convey multiple meanings. The choice of form for expressing an intention depends on the user, whether in speech or writing. With a rich vocabulary and a set of rules, speakers or writers can utilize language flexibly according to their preferences, thereby imparting uniqueness to the language itself. Differences in language usage stem from variations in users' thinking patterns and characteristics. Factors such as age, profession, life experiences, and gender influence how individuals express their thoughts through language. This diversity in language usage is intriguing to examine, particularly for those engaged in the study of linguistics.

For mastering the English language, understanding grammar is crucial. Consequently, grammar forms an integral part of the curriculum in English departments. Proficiency in grammar is necessary for students to construct coherent sentences. Errors in grammar can result in sentences that lack meaning. Furthermore, without a firm grasp of grammar, students encounter difficulties in interpreting sentence meanings. Clearly, this underscores the significance of grammar in language learning.

In any language, sentences hold considerable importance, whether spoken or written. A sentence is always bigger than individual word, phrase or clause. It always begins with a capital letter and ends with full stop or with either exclamation or quotation marks. According to Hogue [1995, 8], a sentence is comprised of a subject and a verb that is able to express a complete idea. Similarly, Azar defines a sentence as a collection of words expressing a statement, question, or command, usually containing a subject and a verb. Additionally, Aldersen [2004, 1] describes a sentence as a complete arrangement of words that conveys meaning. Thus, a sentence can be understood as a group of words with a subject and a verb, expressing a complete thought, a question, or a command.

This article provides an overview of the theoretical framework and reviews of previous research projects. The framework encompasses grammar, clauses, sentences, and sentence types, which contribute to the current study's findings. Notably, multiple theories support the project, drawing from both existing research and previous projects. These previous projects include five analyses. The theories which are used in the article are sourced from papers, journals, and books. They cover various aspects of grammar, sentences, clauses, and sentence types, including simple, compound, complex, and compound-complex sentences.

The overall description of sentence types:

Before delving into compound-complex sentences, let's quickly review clauses and sentence types.

There are two main types of clauses:

1. Independent
2. Dependent

An independent clause is capable of standing alone as a sentence since it contains a subject, a verb, and expresses a complete thought. On the other hand, a dependent clause lacks one or more of these elements and cannot function independently; it serves to enhance an independent clause.

With this understanding, English sentences can be categorized into four types:

1. Simple sentences are constructed from subjects and verbs, with optional additions such as objects, complements, and adverbs [5, 204; Alwi et al., 2003; Oshima & Hogue, 2007]. While Alwi et al. (2003) argue that simple sentences contain only one subject-verb pair (Subject-Verb), Oshima and Hogue (2006) suggest additional patterns like Subject-Verb Verb and Subject-Subject-Verb Verb.
2. Compound sentences consist of at least two simple sentences or independent clauses connected by coordinators (e.g., and, but, so, or) (Oshima & Hogue, 2007). Each independent clause within a compound sentence can stand alone. Moreover, compound sentences contain two or more full predications in the form of independent clauses.
3. Complex sentences merge an independent clause with one or more dependent clauses using subordinators or relative pronouns (Oshima & Hogue, 2007). Among the clauses in a complex sentence, at least one must be independent, while the others are dependent. Both types of clauses contain subjects and predicates, resulting in a complex sentence comprising multiple predications.
4. Compound-complex sentences involve a combination of at least three clauses, including both dependent and independent clauses, connected by both coordinators and subordinators. Additionally, specific comma usage rules apply in complex sentences, such as separating clauses when a dependent clause comes first or when an independent clause comes first. Essentially, a compound-complex sentence integrates features from both compound and complex sentences, often including one or more dependent clauses.

In compound-complex sentences, the arrangement of clauses is flexible and can vary. For instance:

While we can rearrange the order of clauses, it's essential to ensure clarity and effective communication. We should always prioritize arranging sentences based on the importance of the ideas expressed and aim for logical and straightforward communication. In the examples provided, the first one stands out as the clearest due to the sequencing of ideas:

1. The main point: Today is my birthday;
2. The reason: But my best friend is not with me;
3. The explanation: Because she is ill.

Typically, a compound-complex sentence is defined as containing a minimum of 2 independent clauses and 1 dependent clause, overall at least three clauses within the sentence. But, we could further extend this to include four, five or more clauses: “Even though he bought a new car, he kept the old one, and he still drives it because it contains some nostalgic memories.”

In this example, there are four clauses:

1. Even though he bought a new car (dependent)
2. He kept the old one (independent)
3. And he still drives it (independent)
4. Because it contains some nostalgic memories (dependent)

According to Sari, Syarif and Amri (2019) upon identifying students' challenges of writing, it was revealed that many struggled with writing compound-complex and compound sentences. The primary issues encountered by students pertained to grammar/structure and the connectors used to combine two sentences into compound structures. The findings indicated that a significant number of students omitted commas and subjects in their sentences. For instance, instead of writing “People can bring the homework home, and they do it at home,” some students wrote “People can bring the homework home and do it at home,” resulting in incomplete compound sentences. Furthermore, it was noted that students made frequent errors in misinformation, with 10 out of 18 students exhibiting excellent proficiency in crafting compound sentences. Examples included using incorrect verb forms, such as “costing education are paid by government” instead of “costing education is paid by government.” These errors stemmed from confusion regarding subject-verb agreement rules, particularly concerning singular subjects and their corresponding verb forms.

In conclusion, attaining optimal results in writing requires substantial effort, beginning from the early stages of learning. The compound-complex sentence, considered the most intricate form in academic writing, is also notably the least favored by students, as evidenced by research.

The findings indicate that students encounter lots of difficulties when composing compound sentences. Specifically, it was discovered that students struggle with both omission and misinformation of compound sentences. This issue suggests that students face challenges in utilizing punctuation correctly, often employ incorrect connectors, and demonstrate a lack of understanding in grammar, thereby impeding their ability to effectively link independent clauses.

While considering the instructional implications and the impact of teaching on students' comprehension of sentence structure, it is advisable for writing courses to include instruction on different types of sentences. This approach empowers students to diversify their writing by incorporating various sentence structures, ultimately enhancing the overall appeal and engagement of their compositions.