*If I am you, I’d tell him the truth.

[milky]

Why is the present simple not acceptable in (2) but it is in (3)?

1. If I were you, I’d tell him the truth.

2. *If I am you, I’d tell him the truth.

3. If you are a professional singer, I’m Pavarotti.

 

[Francois]

If X, then Y.

Use the present for X if the fact seems possible, from your point of view or from that of the subject of the clause.
Use the subjunctive for X if the fact is very unlikely or not possible (eg. If I were you).
As for Y, it must be in agreement with X from a tense point of view. (2) is clearly incorrect as "If I am you" is impossible, and the tense in "I'd tell him the truth" doesn't fit with the present in the preceding clause.
(3) is correct because the subject may truly believe he's a professional singer. Then, the second clause "I'm Pavarotti" must be in a matching tense, that is, the present (note that you can have other tenses depending on what you want to say, Cf. mixed conditional on this site -- but here the present is clear).
Don't mix up the two clauses X and Y: that's the difference between (2) and (3).

FRC

 

[milky]

If X, then Y.

Use the present for X if the fact seems possible, from your point of view or from that of the subject of the clause.
Use the subjunctive for X if the fact is very unlikely or not possible (eg. If I were you).

Good Francois. Now, our "difficult" student asks what it is about the past simple form, "were", that allows it to be used for unlikely or impossible things. She asks why the present simple cannot be used to show unlikely things.

 

[milky]

If X, then Y.

...(3) is correct because the subject may truly believe he's a professional singer. Then, the second clause "I'm Pavarotti" must be in a matching tense, that is, the present (note that you can have other tenses depending on what you want to say, Cf. mixed conditional on this site -- but here the present is clear).


FRC

Guy trying to chat up girl claims he is Ricky Martin's manager just to impress her:

Girl: Ha! If you are Ricky Martin's manager, then I am Madonna.

 

[Francois]

Now, our "difficult" student asks what it is about the past simple form, "were", that allows it to be used for unlikely or impossible things. She asks why the present simple cannot be used to show unlikely things

Well, what can I say, such are the rules!

Ha! If you are Ricky Martin's manager, then I am Madonna

Yeah, the point seems to be that he might be Ricky Martin's manager, though the following clause shows that the speaker knows better and is clearly being ironic. In other words, the sentence is constructed as if she would admit it was possible, but that is just to make her sarcasm work.
What do you think?

FRC

 

[milky]

Now, our "difficult" student asks what it is about the past simple form, "were", that allows it to be used for unlikely or impossible things. She asks why the present simple cannot be used to show unlikely things

Well, what can I say, such are the rules!

So the "it's arbitrary" approach is all we have?

Yeah, the point seems to be that he might be Ricky Martin's manager, though the following clause shows that the speaker knows better and is clearly being ironic. In other words, the sentence is constructed as if she would admit it was possible, but that is just to make her sarcasm work.
What do you think?

I agree with you analysis. Not here though:

Psychiatrist to patient: If you are Napolean, then I am Caesar, which I'm not, am I?

 

[Francois]

So the "it's arbitrary" approach is all we have?

Languages are not maths :|

Not here though:

Psychiatrist to patient: If you are Napolean, then I am Caesar, which I'm not, am I?

Yes, I believe it's a similar idea, though the purpose is different. Both examples are using absurd reasoning. In the first one, this is in a sarcastic way, and in the second one, that is to make a genuine point to a patient. Both says "Ok, let's suppose X is true, then it leads to Y, which is clearly wrong, so X is wrong too". Whether "X => Y" is true is another debate, but you get the point.

FRC

 

[milky]

So the "it's arbitrary" approach is all we have

Languages are not maths :|

FRC

Indeed not, but how would you tie in all of this to:

The "Proximity and Distance" files.