tense after as if

[bosunyum]

I have some questions about as if.
Recently, I found the following example sentences, which are very confusing for me to understand.

1. He talks as if he is a doctor.
2. he talks as if he were a doctor.
3. he talks as if he had been a doctor.

What are the differences in terms of meaning? Especailly could you be specific about tense?

Also, are the following sentneces acceptable?
4. He looks as if he didn't sleep last night.
5. He looks as if he is tired.
6. he talks as if he has known the fact. Or he talkes as if he knows the fact. (which one is more prefarable?)

i am looking forward to your reply. Thanks!

[Tdol]

1- he could be one
2 he talks, but he isn't one
3 he might be a retired doctor

4 & 5
6- Knows, because he's unlikely to have forgotten or to talk in such a way that would give this idea

[bosunyum]

I have one more question about the sentence;
He talkes as if he had been a doctor.

If we consider the following clause after as if unreal conditional, it is possbile to understand the sentence, like

1. even though he tells (now) that he was a doctor( in the past) it is not true
or we can understand the sentence like
as he was actuallry a doctor( in the past) , he ( now) tells that he was a doctor.
what do you think? Am i right?

[Tdol]

The implication is that he probably wasn't a doctor. He's not saying he's a doctor, but imagine someone elderly giving detailed medical advice- too old to be a doctor, but talking as if he's been one. We're not sure whether he was- the suspicion is that he wasn't. It could be that we know better, but that would come from more context.