1.Incorrect verb forms — 51%
Irregular verb forms are one of the most difficult grammar concepts to master, even for native speakers – many of whom use incorrect irregular forms without realising it. While these “mistakes” are part of English dialects all over the world, these non-standard forms carry a stigma that can significantly damage your credibility if used in formal settings, like business or school. Here are the most common verb conjugation mistakes:
I seen vs. I saw
Incorrect: I seen the movie last week.
Correct: I saw the movie last week.

2 Subject-verb disagreement — 20%
In many languages, it is important that the subject of the sentence aligns correctly with the verb conjugation in terms of number and gender. Since English does not conjugate verbs to reflect the gender of the subject, you only need to pay close attention to the number of the subject – is it a singular or plural noun?
Incorrect: The struggles that the horse experiences while climbing the mountain is intense.
Here the subject the struggles does not align with the verb “is.” Because struggles is plural, the verb should are.
Correct: The struggles that the horse experiences while climbing the mountain are intense.
In English, irregular verbs and compound subjects make subject-verb agreement somewhat tricky. Irregular verbs, like those above, must be memorised, but compound subjects follow a simple rule – they are plural. See below for an example using the compound subject Jane and Mark.
Correct: Jane and Mark are running a marathon this month.

3Run-on sentences — 10%

According to Grammarly’s research, run-on sentences are among the top grammar mistakes made by writers worldwide. A run-on sentence contains two or more independent clauses (a group of words that contains a subject and a verb and that can stand alone as a sentence) that are not connected with correct punctuation. Though there are different kinds of run-on sentence errors, most often writers neglect to use a comma before a coordinating conjunction (and, but, or, etc.).
Incorrect: I enjoy writing immensely but my deadline is looming I am starting to feel overwhelmed.
Correct: I enjoy writing immensely, but my deadline is looming; I am starting to feel overwhelmed.
Each independent clause must be set apart from other independent clauses with punctuation or a comma and conjunction. Punctuation marks that are ideal for marking complete sentences are periods (full-stops), semicolons, and em dashes.

Credit: Grammarly

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