Like vs As if

The word like when used to show comparison, is a preposition, meaning that it should be followed by an object of the preposition but not by a subject and verb. Use as or as if when following a comparison with a subject and verb.

Examples:

You look so much like your mother.
Mother is the object of the preposition like.

You look as if you are angry.
As if is connecting two pairs of subjects and verbs.