Why is 建てる in the result clause in the past tense? The answer from [What is
the correct usage of conditional and
hypothetical?](https://japanese.stackexchange.com/questions/43604/what-is-the-
correct-usage-of-conditional-and-hypothetical) indicates that it should not be
in the past.

> もしも 私が家を建てたなら
>
> 小さな家を建てたでしょう
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2017-02-19T00:52:34.363
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conditionals
Usage of conditional なら
> Conditional clause: もしも 私が家を建てたなら
>
> Result clause: 小さな家を建てた(でしょう)

These Japanese sentences are natural, and we understand that the speaker
didin't (has not) build the small house (or house itself), from the phrae
"建てたでしょう"

Yes "建{た}てた" is the past form of the verb "建{た}てる." The past form of a verb is
used for the situation went unrealized in Japanese.

「もし僕が君だったら、そんなことはしなかった(でしょう)。」= If I were you, I would not do such a thing.
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2017-02-19T02:35:38.860
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This is not "past tense" usage, it is rather more related to **aspect** ( _in
this case whether the speaker considers the action to be complete or
incomplete_ ).

The difference between 建て **た** なら as compared to 建て **る** なら in your example
could be conveyed through the English subjunctive mood like this:

> もしも私が家を建て **た** なら、小さな家を建て **た** でしょう

If I **were to build** a house, I **would build** a small house.

> 私が家を建て **る** なら、小さな家を建て **る** でしょう

If I **build** a house, I' **ll build** a small one.
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2017-02-20T04:39:12.220
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An easy way to understand that usage of なら would be to see it as an imaginary
situation.

> 建てたなら -> If I was in a situation where I built a house.  
>  小さな家を建てたでしょう -> That house would probably be a small one.

It is a way to convey your thoughts/likes/dislikes.  
It basically means that "If I was in situation A, B would be as such because
knowing what kind of person I am, I know I wouldn't have had it any other
way."

This can be used when reflecting on the decision of someone else.  
For your example, it could be that your friend just built a big house, and
then you tell him that if you were in his situation(just built a big house),
it would be a small one.
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