मैं यह वचन नहीं भूलूँगी।

Because the speaker is female and the sentence is in the future tense.

• भूलना = to forget
• भूलूँगी = I will forget said by a female speaker

If the speaker were male, it would be:

• मैं यह वचन नहीं भूलूँगा।

So -ूँगी is a very common ending for first-person singular future, feminine.

It would be:

• मैं यह वचन नहीं भूलूँगा।

The only change is:

• भूलूँगी → भूलूँगा

That is because Hindi verbs often agree with the speaker’s gender in this kind of future form.

In this sentence, वचन means something like promise, vow, or solemn word.

It is a more formal or elevated word than everyday वादा.

So:

• वादा = promise, common in everyday speech
• वचन = promise / vow / pledged word, more formal or literary

That means the sentence has a slightly serious or solemn tone.

Because यह is the direct form of this, and it is used here directly before the noun with no postposition.

• यह वचन = this promise

You use इस when the noun is in the oblique form, usually before a postposition:

• इस वचन को
• इस वचन से
• इस वचन का

So:

• यह वचन = correct here
• इस वचन by itself = not correct in this sentence

In Hindi, नहीं usually comes before the main verb or before the part being negated.

So:

• मैं यह वचन नहीं भूलूँगी। = I will not forget this promise.

This placement is very normal in Hindi.

A rough word order is:

• मैं = I
• यह वचन = this promise
• नहीं = not
• भूलूँगी = will forget

Hindi commonly follows subject–object–verb order, unlike English.

It comes from the verb भूलना (to forget).

A simple way to understand it:

• dictionary form: भूलना
• future stem: भूलूँ-
• feminine ending: -गी

So:

• भूलूँगी = I will forget (female speaker)

This is one of the standard future forms in Hindi.

A helpful pronunciation is:

• bhū-lū̃-gī

Broken down:

• भू = bhū
• लूँ = lū̃ (nasalized)
• गी = gī

The ँ shows nasalization, so लूँ is not just loo, but a nasalized lū̃.

Because the future form भूलूँगी already carries the needed information.

In present tense, Hindi often uses a participle plus a form of होना:

• मैं भूलती हूँ = I forget / I am forgetting

But in the future, the verb itself usually shows the future meaning:

• मैं भूलूँगी = I will forget

So you do not need हूँ here.

It sounds a bit formal, serious, or even literary, mainly because of वचन.

A more everyday version might be:

• मैं यह वादा नहीं भूलूँगी।

Both are correct, but:

• वचन = more solemn, formal
• वादा = more everyday

Yes, in other contexts वचन can have other meanings, such as word, statement, or even a grammatical term like number in grammar discussions.

But in a sentence like:

• मैं यह वचन नहीं भूलूँगी।

the natural interpretation is promise or vow, not a grammar term.

Context decides the meaning.

Very roughly, it is:

• मैं = I
• यह = this
• वचन = promise / vow
• नहीं = not
• भूलूँगी = will forget (female speaker)

So the structure is basically:

• I this promise not will-forget

That sounds odd in English, but it reflects normal Hindi word order: subject + object + negation + verb.