When do you use "as" and when do you use "as if" in Adverbial clauses of manner?

1 个月前
Hi, everybody!

I don't understand when in Adverbial clauses of manner I could use "as" without "if". The examples from the book I have don't help much. They are "The film depicts life as it was in 1900." and "Complete each exercise as I showed you."So, in this sentences it IS possible to use only "as" without "if".
However, in "Can I help you? You look as you're lost?" "as" can't be used without "if".
Pay attention I am talking only about Adverbial clauses of manner.

Thanks a lot!