Student or Learner
I am wondering if one can use both "was" and were" in this kind of sentence.
1. Tom is lazy. If Tom was not lazy, he would have already finished his university.
2. Tom is lazy. If Tom were not lazy, he would have already finished his university.
Does one can use "was" only in informal English, or it could be used also in formal English?
It's a question of the use of the subjunctive mood or not. Usage of the subjunctive varies, it is much more common in AmE than in BrE.
I would use "were" (the subjunctive) if it was felt that Tom would always be lazy and there was no hope of him changing.
I would use "was" if I felt that he could change.
We don't speak of people "finishing their university" in AmE. People finish their degrees.