As if or As though

1. As if / as though + present simple:

It is used to express something or someone seems like. For examples,
It looks as if it might rain. [The sky turns dark, and it will possibly rain.]
He works on the computer as if/though he's skillful. [He can do things, and he probably can fix it.]

2. As if / as though + past simple:

It is used to express something is presently unreal. For examples,
She tells me as if/though she knew the story. [She obviously doesn't know the story.]
He walks as if/though he were an old man. [But in fact he's a young man.]
Compare the following two sentences:
He looks as if/though he's sick. [He is sick.]
He talks as if/though he were sick. [He's actually well.]

3. As if / as though + past perfect simple:

It is used to express something was unreal in the past. For examples,
He looked as if/though he had done something. [He didn’t do anything wrong.]
They responded as if/though the problems were not serious. [The problems were serious.]