[masaegu]

Here is the rest.

31. Grammatical but unnatural.
We do not really say いい学生 in this context but then anything we would say naturally would not even look like the English sentence, so let us leave it as is.

(For the non-beginning learner, we say 「よくできる学生」 or 「優秀（ゆうしゅう）な学生」)

32. に　＞＞　を
The rest is super-natural.

35. Not quite (though that may be how they teach you to say it.)
「日本語の勉強はやさしいです。」

37. が　＞＞　は

39. Colloquially, correct. But I would imagine that, at your level, you would be required to use か in every question.　いる　＞＞　いますか

42. Same problem with # 39