No quiero volver a engordar.

Volver a + infinitive usually means “to do something again”.

• volver on its own = to return, to go back
• volver a engordar = to get fat / gain weight again

So:

• No quiero volver a engordar.
  = I don’t want to get fat again / I don’t want to gain weight again.

This structure is very common:

• volver a intentarlo – to try again
• volver a llamar – to call again
• volver a verlo – to see him/it again

In this pattern, volver almost always takes a before another verb in the infinitive:

• volver a + [infinitive]

The a is a required preposition that links volver with the action that’s being repeated.

Correct:

• volver a comer – to eat again
• volver a estudiar – to study again
• volver a engordar – to get fat again

Incorrect:

• ✗ volver engordar (missing the a)

Yes, it’s correct and very natural:

• No quiero engordar otra vez.
• No quiero engordar de nuevo.

These also mean I don’t want to get fat again.

Nuance:

• volver a engordar puts a little more focus on the idea of returning to a previous state.
• engordar otra vez / de nuevo is more neutral, just “to get fat again”.

In everyday speech, all of these are fine and very commonly used. There’s no big difference in politeness or formality.

Yes, engordar is a verb that comes from the adjective gordo (fat).

• gordo / gorda – fat (adjective)
• engordar – to get fat, to gain weight (verb)

Examples:

• Estoy engordando. – I’m gaining weight.
• No quiero engordar. – I don’t want to gain weight.

So the sentence literally means:
No quiero (I don’t want) volver a engordar (to get fat again).

Both engordar and engordarse exist and are used.

• engordar (non‑reflexive) – very common, especially when talking in general about gaining weight.
• engordarse (reflexive) – also used, sometimes with a bit more emphasis on the person undergoing the change.

You can hear:

• No quiero volver a engordar.
• No quiero volver a engordarme.

Both are understood as I don’t want to get fat again. In Latin America, the non‑reflexive engordar is extremely common and sounds perfectly natural here. The reflexive form is not required in this sentence.

In Spanish, the normal rule is:

no + [conjugated verb]

So:

• No quiero volver a engordar.
  (I don’t want to get fat again.)

We usually don’t say:

• ✗ Quiero no volver a engordar.

That last version is grammatically possible, but it sounds more marked or “philosophical” (like I want not to get fat again, focusing on the wanting not). Everyday Spanish just negates the main verb:

• No quiero… (I don’t want…)

No. Spanish word order is more flexible than English, but not that flexible.

In this kind of sentence, you should keep:

1. No (negation)
2. quiero (conjugated verb)
3. volver a engordar (infinitive phrase)

Correct:

• No quiero volver a engordar.

Incorrect:

• ✗ No volver a engordar quiero.
• ✗ Quiero volver a engordar no.

The conjugated verb (quiero) almost always goes right after no in simple statements.

In Spanish, when one verb expresses desire, ability, obligation, etc., and it’s followed by another verb, the pattern is:

• [conjugated verb] + [infinitive] (+ [infinitive])

Here:

• quiero – first person singular, present tense (I want)
• volver – infinitive (to return / to… again)
• engordar – infinitive (to get fat / to gain weight)

So literally:

• No quiero – I don’t want
• volver – to return (i.e., to do again)
• a engordar – to get fat

Similar patterns:

• No puedo dormir. – I can’t sleep.
• Quiero estudiar español. – I want to study Spanish.
• Necesito empezar a trabajar. – I need to start working.

Engordar is not automatically rude; it’s the standard verb for to get fat / to gain weight and is widely used.

However, when talking about other people, you might choose more neutral or softer expressions, like:

• subir de peso – to gain weight
• aumentar de peso – to gain/increase in weight

Examples:

• He subido de peso últimamente. – I’ve gained weight lately.
• No quiero subir de peso otra vez. – I don’t want to gain weight again.

But No quiero volver a engordar is absolutely normal when talking about yourself.

The difference is who is supposed to avoid gaining weight.

1. No quiero volver a engordar.
   – The speaker is the one who might gain weight again.
   – I don’t want to get fat again.

2. No quiero que vuelva a engordar.
   – The subject of vuelva is someone else (he, she, you, it).
   – I don’t want him/her/you/it to get fat again.

Grammar point:

• When the same subject does both actions, Spanish uses infinitive:
  No quiero volver a engordar. (I don’t want to X.)

• When there are two different subjects, Spanish uses que + subjunctive:
  No quiero que él vuelva a engordar.
  (I don’t want him to X.)

Yes, and it sounds natural and maybe a bit more neutral/polite:

• No quiero volver a subir de peso.
• No quiero volver a aumentar de peso.

Both mean I don’t want to gain weight again, without directly using the idea of “getting fat”. In Latin America, subir de peso is very common and a bit more “objective” than engordar, which can feel more physical/direct.