my common mistake

Article mistakes are my common mistakes in my written assignments. Sometimes I can't be sure which is true. For example: I say that 'I am an student', but it is not true. It can be 'I am a student' This is only simple example. But article mistakes are big problem for us. Because we often use them. I don't want to make them again so I have studied it from Grammer book for a week. Also I read many example sentences.