Is it possible to change です to でした in ～んです grammar?


Is it possible to change

> このネックレスは買ったんですか。

to

> このネックレスは買うんでしたか。

without changing the meaning?




No, these two sentences are different, and you have to use the first sentence
in 99% of cases.

The first one is easy:

> このネックレスは買ったんですか。  
>  Is this necklace what you bought? / Did you (really) buy this necklace?

The second one is difficult:

> このネックレスは買うんでしたか。  
>  Is this necklace what you would buy (instead of just looking or borrowing)?  
>  Are you thinking of (actually) buying this necklace?

This is an euphemistic version of このネックレスは買うんですか. Sometimes the past tense is
used to express euphemism, just like in English. This sentence is not very
polite and sounds like a broken
[マニュアル敬語](https://japanese.stackexchange.com/a/18255/5010) to me, but a native
speaker may say this at a shop to someone who is a bit higher than him.




No, these two sentences are different, and you have to use the first sentence
in 99% of cases.

The first one is easy:

> このネックレスは買ったんですか。  
>  Is this necklace what you bought? / Did you (really) buy this necklace?

The second one is difficult:

> このネックレスは買うんでしたか。  
>  Is this necklace what you would buy (instead of just looking or borrowing)?  
>  Are you thinking of (actually) buying this necklace?

This is an euphemistic version of このネックレスは買うんですか. Sometimes the past tense is
used to express euphemism, just like in English. This sentence is not very
polite and sounds like a broken
[マニュアル敬語](https://japanese.stackexchange.com/a/18255/5010) to me, but a native
speaker may say this at a shop to someone who is a bit higher than him.

