You will see many stars is the sky-----you come out at a fin

应该是用WHEN啊,它引导时间状语啊
if引导条件状语从句,when引导时间状语从句!
先去看看什么叫从句 吧
~~~~~~~~~~~~ 条件状语从句是表示主句动作发生的前提或条件的从句.引导条件状语从句的连词有if, unless, so/as long as, as/ so far as, on condition that, in case, suppose, supposing等.条件状语从句中的谓语动词一般要用一在时或过去时代替一般将来时或过去将来时.
if
if表示正面条件,意为“如果”.
If you ask him, he will help you.
如果你向他请求,他会帮助你.
Difficulties are nothing if we are not afraid of them.
如果我们不怕困难,困难就算不了什么了.?