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Hello,

A. Isn't it if you tell the truth, you will have a peace of mind?
B. Isn't it when you were 16, you had a crush on her?
C. Isn't it that all men are equal?

1. Is the phrase "Isn't it if/when/that..." as used in my examples above natural?
2. In B, can I also used "Wasn't it when...?" since the tenses of the verbs that follows are in the past?
3. In A and C, can I add 'that' before 'if' and 'when' respectively?

Please advise. Thanks.
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1. I don't find 'isn't it' natural in any of those sentences without 'that':

A. Isn't it that f you tell the truth, you will have a peace of mind?
B. Isn't it that when you were 16, you had a crush on her?
C. Isn't it that all men are equal?

2. In B, can I also used "Wasn't it when...?" since the tenses of the verbs that follows are in the past?-- Yes

3. In A and C, can I add 'that' before 'if' and 'when' respectively?-- You must, in my opinion.
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Thanks, Mister M.
Mister Micawber2. In B, can I also used "Wasn't it when...?" since the tenses of the verbs that follows are in the past?-- Yes
What is the difference in meaning if changed to 'wasn't"?
Which is more commonly said by native speakers if the meaning is the same?
The verb has a tendency to regress with the perspective of the statement, so native speakers tend to use that; but the assessment of the action observed then is still true, so the present tense is not wrong, and might be the preferred choice, say, in cross-examination in a court trial.
That makes sense to me now. Thank you for your explanation.
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