Influenced by their mother language, so they commonly mistake the punctuation rules of starting with capitals and ending with full stops.In addition to the grammatical mistakes they usually make with tenses ,subject verb agreement,etc.Last to mention is the spelling mistakes.

One has to bear in mind that producing discourse he needs to be grammatically, socially and strategically correct. Most of the students focus on being grammatically correct only.

Vilemar Magalhães

Hello. It depends on the students' first language. In any case, apart from influences from the students' first language, common mistakes concern the use of the definite article ("the"), differences in the use of the present perfect vs. simple past or present simple, or the terminal -s in the third person of the simple present tense (see Seidlhofer 2004: Article Research Perspectives on Teaching English as a Lingua Franca

If we take only grammar as the main point of our discussion, phrasal verbs, and false friends are the most challenging aspects of communicating in a second language.

Vilemar Magalhães

The errors can be caused by various factors, including inadequate understanding of grammar rules, limited vocabulary, lack of practice, poor proofreading habits, and insufficient familiarity with academic writing conventions. Regular practice, feedback from instructors, and utilizing writing resources can help students improve their writing skills.

Grammatical Errors:

Subject-Verb Agreement: Students may struggle with ensuring that subjects and verbs agree in number.

Tense Consistency: Switching tenses inappropriately within a piece of writing.

Incorrect Use of Articles: Misuse of "a," "an," and "the."

Punctuation Errors:

Comma Splices: Using a comma to join two independent clauses.

Run-on Sentences: Failing to separate independent clauses correctly.

Misplaced or Missing Commas: Misplacing commas in complex sentences or omitting them where needed.

Spelling Errors:

Homophones: Confusing words that sound the same but have different meanings (e.g., "their," "there," "they're").

Typos and Misspellings: Simple errors due to lack of proofreading.

Vocabulary and Word Choice:

Incorrect Word Usage: Using words inappropriately based on context.

Redundancy: Repeating the same idea or using unnecessary words.

Sentence Structure Errors:

Fragmented Sentences: Writing incomplete sentences that lack a subject or a verb.

Awkward Constructions: Creating sentences that are confusing or hard to follow.

Coherence and Cohesion:

Lack of Logical Flow: Failing to organize ideas in a logical sequence.

Weak Transitions: Poor use of transition words and phrases to connect ideas.

Stylistic Issues:

Overly Complex Sentences: Writing sentences that are too long and complicated.

Inappropriate Tone: Using a tone that doesn't suit the context or audience.

Content and Argumentation:

Weak Thesis Statement: Presenting a thesis that is not clear or compelling.

Insufficient Evidence: Failing to support arguments with adequate evidence.

Referencing and Citation Errors:

Plagiarism: Not properly citing sources.

Incorrect Citation Format: Misapplying citation styles such as MLA, APA, or Chicago.