Grammar in ACCA exams

I am not a native English. My English writing is very very bad. I always get a lot of mistakes when I write in english, even a simple one. Now the question is:

If I make a mistake in grammar or in vocabulary when I do the ACCA exams, do they affects my score ???? For example: Actually I get 80, but because I have a lot of mistakes in my grammar and vocabulary, the examiner will reduce my score into 75 or 70.

And also please help me, how can I improve my English writing in the ACCA exam…..