I think every learner is troubled by this use of "as if" and I totally sympathise with bosunyum because this question troubles me too.
According to what you have both said, I draw my own conclusion and please tell me if I'm right.
1. He talks as if he is a teacher
(it is very possible that he is a teacher now because he behaves like one)
2. He talks as if he were a teacher
(He is NOT a teacher, but he behaves like one)
3. He talked as if he were a teacher
(a. It was possible that he worked as a teacher at the moment of speaking --which is past;
b. he behaved like one but was NOT a teacher then)
4. he talked as if he had been a teacher
(a. It was possible that he had worked as a teacher before the "talking"
b. He behaved like someone who had been a teacher before the moment of speech --which is already past-- but in reality he had never been a teacher before)
It's a little tricky, since "as if" is often ambiguous. Consider this example:
1. He talks as if he's an expert.
Here are two possible contexts:
"I see that José is giving his opinion on the Champions' League final yet again."
"I know. Tedious, isn't it?"
"Very tedious. And the worst thing is, he talks as if he's an expert..."
"When of course he knows absolutely nothing about the game..."
— i.e. José is attempting to present himself, falsely, as an expert.
"Did you read José's post about Arsenal vs Barcelona on Wednesday night?"
"I did indeed."
"I was quite surprised. I didn't know he was an expert on football."
"Nor did I. But he certainly talks as if he's an expert..."
— i.e. José resembles an expert, when he talks about the football; and he may well be one.
In everyday conversation, the different stress pattern (see the italics in #3) distinguishes the two meanings. But in written English, it's not always possible to limit the meaning to one or the other, without the context!
Does that make it any clearer? (Now that it's more complicated...)
Yes it does! But isn't the second interpretation of "he talks as if he is an expert" used only in speech and informal writing; whether the one with the subjunctive interprets better the unreality of a present situation in formal writing?
And what do you think of my interpretations of the all the other possibilities, i.e. #3,4 & 5 ?
I can't speak for other kinds of English; but I don't find a clearcut formal/informal difference in the usage of ordinary BrE-speakers. It seems to me that people reach for the subjunctive (or a past tense form) when they want to express "remoteness", and the indicative when they want to express "immediacy"; but beyond that, I wouldn't like to say. (Though of course, some speakers never use a subjunctive; and some speakers are extremely conscious of using the subjunctive.)
For instance, if you listen to a phone-in, you will hear subjunctives (and subjunctive inversions such as "Were you to XYZ...") from the kind of people the textbooks say don't use subjunctives; and if you listen to interviews with national political figures, you will hear indicatives where subjunctives are supposed to prevail.
It would be much easier if it was clearcut...
PS: Your interpretations of #3 and #4 look fine to me, but I can't find #5. :S