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Merry Christmas:

A is talking to B about C. C told to A something about B earlier and wouldn't like A to tell B, but A just did.
A to B:" I'd really appreciate it if you didn't let C know. please."

What is A using didn't. What's the difference if A uses "...if you don't let C know"?

Thanks,

Ench
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Correct tenses in conditional sentences:
I'd appreciate it if you didn't tell...
I'll appreciate it if you don't tell...

Unfortunately, however, many native speakers don't follow these sequences, so you might well hear: I'd appreciate it if you don't tell.
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Yoong LiatIs the first request more polite than the second?
Oh, yes! Definitely! "I'll appreciate it if ..." sounds almost commanding! Emotion: smile

Compare:

I'll thank you to keep your hands off my things!<said in a very haughty manner>

Or

I'll thank you to keep your nose out of my business! <same>

CJ
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Comments  
PhilipCorrect tenses in conditional sentences:
I'd appreciate it if you didn't tell...
I'll appreciate it if you don't tell...

Unfortunately, however, many native speakers don't follow these sequences, so you might well hear: I'd appreciate it if you don't tell.

Is the first request more polite than the second? Many thanks. Emotion: thinking
 CalifJim's reply was promoted to an answer.
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EnchantedWhat Why is A using didn't. ?
Because of the would (I'd = I would) and the if. In this pattern, the past tense does not mean anything past. It's used, more or less, to create tense agreement with would.

... would ... if ... past

I would appreciate it if you didn't let C know.
The birds would fly away if you opened the cage.
You would be able to reach the top shelf if you were just two inches taller.

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EnchantedWhat's the difference if A uses "...if you don't let C know"?
It's a mistake. In the sentence you cited, it's not a big mistake, but sometimes the use of the present tense with would can be very jarring.

CJ