English and ESL Instructor
Incorrect: I am a student. And my favourite subject is English.
In a previous post that I wrote about comma faults, I talked about independent and dependent clauses. Whenever there is a conjunction at the beginning of a clause, the clause is always a dependent one. Notice that the second sentence beginning with "and" is a dependent clause, so it cannot actually stand alone as a sentence.
If we were to fix the example above, we would use a comma instead of a period.
Correct: I am a student, and my favourite subject is English.
Here is one more example:
Incorrect: Jack is of Chinese descent. But he does not understand Chinese.
Correct: Jack is of Chinese descent, but he does not understand Chinese.