Is"...as if you were __" grammatically correct?

RommelIs "...as if you were __" grammatically correct in the following sentence?

Yes.

Hello, Mister Micawber, could we also say "you look like you are (for present meaning) / were (for past) not an ophthalmologist", please?

Also, does the structure as if + past refer to the past or does it refer to the present?

For e.g. He acts as if he knew everything (is it a correct sentence? Does it refer to the past or present?)
He

Laboriouscould we also say "you look like you are (for present meaning) / were (for past) not an ophthalmologist"

That suggests that he is really not an ophthalmologist; your original suggests that he is.

Laborious does the structure as if + past refer to the past or does it refer to the present?

The present.

Mister Micawber, what then is the difference among the following sentences, aside from the tenses of the verb used in "as if he __"? How very confusing!

He acts as if he knows everything.
He acts as if he knew everything.
He acts as if he had known everything.

He acted as if he knew everything.
He acted as if he had known everything.

He acts as if he knows everything.-- It is more likely he knows everything
He acts as if he knew everything. -- It is less likely he knows everything -- These 2 sentences are often used interchangeably, with the 2nd the more strictly correct.

He acts as if he had known everything.-- Odd. He knew something in the past and now he acts like he knew it then.

Bravo, Mister Micawber! Your explanations are crystal-clear.

RommelBravo, Mister Micawber! Your explanations are crystal-clear. How I wish I were grammatically wise like you!

Hmm. I thought I was waffling. I'll bet another member could clean up my mess.