The purpose of academic writing, as with most other kinds of writing, is to communicate. For you, as a student, your writing is the marker's only window to your thoughts. Therefore it is important that you learn how best to write in a way which will convince the marker that you understand what you are talking about.
A clearly written assignment lets the thought show through. A muddy one obscures the thinking, and gives the impression the thought-processes are muddy too. Use quotations and paraphrase sparingly because the marker cannot tell from other people's words what you are thinking. Get into the habit of using your own words. Like physical exercise, it may be hard work at first but you will get better and better at it until you have a good strong intellectual physique! To take the short cut of relying on other peoples' words bypasses thought processes and will make the material very hard to remember as it has not been assimilated into your own mind - it is not your possession. You are not primarily a collector of information, a recorder of the wisdom of others - that would be very boring. You are an active learner and a unique interpreter of the information which comes your way.