... until now

[Leong187]

1) Might I enquire you why you have not mentioned until now?
2) I have never felt so embarrassed in my life!

Does it make any difference if I replace the present perfect tense with the past perfect tense in the sentences above?

Thanks in advance.

[Maryam90]

Past perfect is used for an action that happened in the past.so you can't use it in the first sentence because of "until now".
I have never felt so embarrassed in my life. you express that you have this feeling so past perfect isn't correct.
However,I am just a student .I'm not sure.

[Beeesneees]

If you replace present perfect with past perfect you will simply put the actions in the past.

Might I enquire you why you had not mentioned it until now? - indicating that it has recently been mentioned, but not as recently as 'have not'. It's unusual to see this form with 'until now'. It doesn't match well.
I had never felt so embarrassed in my life! - recounting a past embarrassment.
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Cheers m' dears!