Which is more correct to combine the following two sentences with 'if': first or second conditional?
He won't study hard, so he won't get high marks.
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
The first conditional is appropriate here:
If he doesn't study hard, he won't get high marks.
(It would also be possible to say "If he won't study hard, he won't get high marks". Here, "won't" is best understood not as a reference to future time but to willingness = "if he isn't willing to study hard".)
The second conditional is possible but changes the meaning:
If he didn't study hard, he wouldn't get high marks.
His failure to study hard becomes a hypothetical. There is now an implication, assumption or suggestion that he will study hard, which contradicts your original sentence ("He won't study hard...").
Second conditional is the only one that can convey that meaning.
A first conditional sentence combining those two clauses looks like this:
If doesn't study hard, he won't get high marks.
The problem is this sentence allows the possibility that he might study, whereas your original sentence does not. It states he will not study and as a result will not get high marks, so first conditional cannot work.
A second conditional version looks like this:
If he studied hard, he could get high marks.
As with the original, this sentence means he will not study hard and therefore will not get high marks. I used "could" rather than "would" because the original doesn't suggest he would certainly get high marks if he studied, just that he won't get high marks because he won't study.