If "unless" implies a condition for something *not* happening:
* If
* When
* Provided that
* As long as
Example:
* Unless you study hard, you won't pass the exam.
* If you study hard, you will pass the exam.
If "unless" implies a condition for something *happening*:
* Only if
* Except if
* But for
Example:
* You won't get a promotion unless you improve your performance.
* You will get a promotion only if you improve your performance.
Other possibilities:
* Therefore
* Consequently
* So
These words emphasize the consequence of something happening, whereas "unless" implies a condition for something *not* happening.
Ultimately, the best opposite word for "unless" depends on the specific sentence and intended meaning.