Conjunctions

If, when and unless

If, when, unless + present simple express future time.

If = perhaps it will happen.
When = it is definitely expected to happen.
Unless = if ... not (it will happen except if).

• The dog won't bite if you are gentle.
• The dog won't bite when it is asleep.
• The dog won't bite unless you frighten it. (= if you do not frighten it)