As if and as though mean the same. We use them to say what a situation looks like.

She spoke to me as if she knew me, but I had never met her before. OR She spoke to me as though she knew me, but I had never met her before.
It looks as if / as though it is going to rain.
She cried as if / as though she was dying.

After as if and as though we often use a past tense with a present meaning. This shows that the comparison is unreal. A present tense, on the other hand, shows that we are talking about real and possible situations.

She talks as if / as though she knows everything. (Perhaps she knows everything.)
She talks as if / as though she knew everything. (But she doesn’t.
They look as if / as though they know each other. (Perhaps they know each other.)
They look as if / as though they knew each other. (But they don’t know each other.)

Notes

When the main clause is in the past tense, we do not use a past perfect after as if / as though to show that a comparison is unreal. Instead we use a simple past in both clauses.

He looked as if / as though he knew everything, but he didn’t. (NOT She looked as if / as though she had known everything.)

Were instead of was

In an informal style, were is used instead of was in an unreal comparison. This is normal in American English.

He looks as if he was rich. OR He looks as if he were rich.