I am already in college, but I'll make the ENEM anyway.
You need an adversative conjunction (but – it will fit perfectly) in your sentence. Why? Because if you are already in college, you don’t need to sit for the ENEM.
The ENEM is a really hard exam, even if you study a lot, you'll find some difficulties ( to develop) in this exam, it's pretty extensive and we don't have much time to stop and think(ing).
Be careful with collocations: you can’t say “difficulties develop”. You can say: … you’ll find some difficulties in the exam…. Or you can also say: even if you study a lot, some potential difficulties arise…..
You can also choose another verb before the noun difficulty and therefore much more common in everyday use:
Encounter difficulty, experience difficulty, face difficulty, get into difficulty, have difficulty, run into difficulty…..
(Sometimes) Some questions (look like- we use this verb for a person to mention the appearance of his or her, e.g. My brother looks like my mother, ok? the correct verb here is to seem ( they goes by opinion from who's doing the exam, I mean, there is no the exact answer. – too much Portuguese interference here.
Some of the questions seem to be interpretive ones; the answers can vary from person to person.
Math is something that requires (Math is just one subject the verb need an s – Math and Physics require …. Two subjects no s) a good memory and (good notion of how to develop the questions.) an intense concentration to come up with the answers (if you are talking about how to solve the questions of an exam, focus on the way you are going to say that, ok?).
Chemistry isn't hard, Biology ( neither) either (negative verb positive conjunction). Of course, the likes or dislikes of school subjects ( depends (by- ) on (correct preposition) depends on the person, to me (the more) the most difficult( is) are Literature and Math.
It’s better if you use: the likes or dislikes of school subject can differ from person to person (instead of the verb depend)
( redaction-redação de escola, prova :essay) The essay requires a good development plan. (of how we elaborate our opinion about something and how we organize - too much Portuguese here).
My suggestion: The essay requires a good development plan. We should write clearly and construct our sentences carefully. It’s important to capture and retain the reader’s interest.
( The majority of the times- wrong choice of word ) Most of the times the approval of this exam comes by luck, if the ENEM (have) has questions related ( about ) to what we've ( been ) studied.http://dictionary.cambridge.org/pt/dici ... ugues/seemhttp://dictionary.cambridge.org/pt/dici ... ues/look_1