if 可以省略they are吗,就像while（they are）good这样?相信谁

if引导状语从句，虚拟语气，都可以用省略形式。
但是这种省略是有条件的，即：
if 状语从句的主语必须和主句与相同。例如：
【These apples主句主语】 are nice to eat if 【they从句主语】 are ripe.
=These apples are nice to eat if ripe.
You can eat these apples if they ripe.
=These apples can be eaten if ripe.
【Tom】 will certainly help us if 【he】 is here.
=Tom will certainly help us if here.
祝你开心如意！

不能。这是虚拟句型,表示与现实相反的事.
If sb were ....,sb would do sth..

可以的,但要在对方明确你所指的东西时.
比如You can take them home if they are clean.= You can take them home if clean.