Will

No, it's not correct because the speaker refers to what happened in the past or something that is not true now.
It was a moment he would want to forget. (Sounds rather unnatural to me)
It was a moment he wished he could forget. (Better)

SicaNo, it's not correct because the speaker refers to what happened in the past or something that is not true now. It was a moment he would want to forget. (Sounds rather unnatural to me) It was a moment he wished he could forget. (Better)

Thanks for the reply. But still, is the It was a moment he will want to forget ungrammatical?

Yes, it is, in my opinion. If what you imply is that he will want to forget a moment that happened before, you can simply say, "He will want to forget that moment".

Anonymous But still, is the It was a moment he will want to forget ungrammatical?

It is grammatical.

ozzourti Anonymous But still, is the It was a moment he will want to forget ungrammatical?It is grammatical.

Would you mind telling me the reason, please? Yet I'm still learning here, I would be happy to learn from others

There are a couple of rhetorical devices in use here that you may encounter in English from time to time.
First of all, the reversal of the expected word order places extra emphasis on the moment, telling us that this moment was particularly noteworthy.
Secondly, there is deliberate understatement in the phrase "he will want to forget". The implication is that he will not be able to forget

SicaWould you mind telling me the reason, please?

A better question would be "Why do you think it is not grammatically correct?" Something happened in the past that 'he' wants (now) or will want (in the future) to forget.

SicaIt was a moment he wished he could forget. (Better)

That's acceptable, but it changes the meaning.

Sica ozzourti Anonymous But still, is the It was a moment he will want to forget ungrammatical?It is grammatical.Would you mind telling me the reason, please? Yet I'm still learning here, I would be happy to learn from others

It was a moment he will want to forget has an anticipatory subject it, a copula was and a postponed (real) subject,

Anonymousa noun a moment embedded in the noun phrase a moment he will want to forget.

T