Avoid ‘we’, because it is often unclear who ‘we’ refers to. You and your marker? You and a co-author? The whole world???
It is also very bad style to refer to yourself as ‘we’—writers sometimes do it when they have problems with their essay and they want to ‘distance; themselves from it, saying in effect, ‘Do not blame me for this result’.
Avoid ‘we must teach like this’ or ‘we should concluded that…’. If you do this, you are making two mistakes. First, you are telling your marker what to think and do (not a good idea) and second, you are limiting your own position. You are saying ‘things are defined and clear cut’, whereas they are often NOT!
Avoid ‘you’. It is too personal and conversational. That the ‘expert talking to learner’ style used in textbooks and handbooks is NOT appropriate in formal writing, since you are still a learner.
You can use ‘I’ at several points, but making certain the sentence refers to something you are, as a student and as the author of the assignment, responsible for. Thus you could put ‘I shall divide the argument into two main sections’ (you could also write ‘the argument is divided into two main sections’). On the other hand, you do not want to write, ‘I have argued elsewhere that adults learn grammar faster than children’, as this makes you sound like a world expert with 30 books published!
Try and avoid ‘I think X is a good idea’. We want you to argue the case WHY X is or is not appropriate, rather than simply assert(=state) that you think it is. This is one of the big differences between conversation and university assignments. In an assignment, you have to make sure you give the evidence for opinions and you need to use impersonal verbs like ‘seem’.
Thus you would write, ‘Smith would seem to be wrong in arguing that…’ or ‘This seems incorrect’, rather than ‘I think Smith is wrong’.