Only the second one is correct - "Unless he tells the truth, I will leave him." For the others, you could say: "He must tell the tells the truth; otherwise I will leave him" "Either he tells the truth, or I will leave him."

I just want to add that "unless he tells the truth" = "if he does not tell the truth". Literally, "unless..." means "if not that...", but it would be a bit confusing to say "if not that": "If not that he tell the truth, I will leave him" (note: "tell", not "tells"). The reason "unless" is a confusing word to learn is that it deals with negatives. Negatives are always a source of confusion in languages.

The correct sentence is: "Unless he tells the truth, I will leave him." This sentence effectively conveys the conditional relationship between the truth-telling and the speaker's decision to leave. The other options have minor issues in terms of grammar and logic.

unless he tells the truth i will leave him.

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