While: Indicates a contrast or simultaneous occurrence.
Example: He read a book while waiting for the train.

Since: Specifies a starting point in time or a reason.
Example: He has been studying English since last year.

Unless: Introduces a condition that must be met for something else to happen.
Example: Unless you finish your homework, you can't go out to play.

Provided that: Introduces a condition or requirement.
Example: You can borrow my car provided that you return it by tomorrow.

Therefore: Indicates a conclusion or result.
Example: He studied hard; therefore, he got good grades.