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.
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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...").