Learn Hardcore Greek: Θέλω να τρώω λιγότερα γλυκά, γιατί δεν θέλω να πάρω κιλά. - I want to eat fewer sweets, because I don’t want to gain kilos.

Modern Greek does not really use an infinitive (a form like to eat, to take).
Instead, after verbs like θέλω (I want), Greek uses the particle να plus a verb in the subjunctive mood.

So:

Θέλω να τρώω ≈ I want to eat
Θέλω να πάρω ≈ I want to gain / put on

English uses an infinitive (to eat, to gain), while Greek uses να + subjunctive for the same idea.

What is the difference between να τρώω and να φάω?

Both come from the verb to eat, but they express different aspect (kind of action):

να τρώω – imperfective aspect: ongoing, repeated, or habitual action
- Θέλω να τρώω λιγότερα γλυκά = I want to be eating fewer sweets / I want to eat fewer sweets (as a habit).
να φάω – aorist aspect: single, complete action
- Θέλω να φάω λιγότερα γλυκά would suggest something like I want to eat fewer sweets (on this particular occasion / in this one instance).

In your sentence, να τρώω is used because it talks about a general, ongoing habit: eating sweets in your life, not just one specific time.

Why is it λιγότερα γλυκά and not λιγότερο γλυκά or λιγότερες γλυκά?

In Greek, adjectives must agree with the noun in gender, number, and case.

γλυκά here is a neuter plural noun (accusative plural).
So λιγότερα (comparative of λίγα, few) is also neuter plural accusative, to match γλυκά.

That’s why we say:

λιγότερα γλυκά = fewer sweets
not:
λιγότερο γλυκά (neuter singular – wrong here)
λιγότερες γλυκά (feminine plural – wrong gender)

What exactly does γλυκά mean here? Is it an adjective or a noun?

The basic adjective is γλυκός, -ιά, -ό = sweet.

το γλυκό (neuter singular noun) often means a dessert / a sweet / a pudding.
τα γλυκά (neuter plural noun) means sweets, desserts, sweet things, candy, pastries in general.

So in λιγότερα γλυκά, γλυκά functions as a noun: sweets or desserts, not just the adjective sweet.

What tense or form is τρώω here? It looks like the present; how is it subjunctive?

Formally, the subjunctive imperfective of many verbs looks the same as the present indicative in the 1st person singular:

Present indicative: (εγώ) τρώω = I eat / I am eating
Subjunctive (with να): να τρώω = (in order) that I eat / to eat

You know it’s subjunctive in this sentence because it is introduced by να and follows the verb θέλω:

Θέλω να τρώω = I want to eat / I want to be eating
Not just a statement of fact, but a wished-for action.

Why is the first verb να τρώω but the second one να πάρω and not να παίρνω?

Again, it’s about aspect:

να τρώω (imperfective) – describes an ongoing, repeated habit: to be eating less sweets in general.
να πάρω (aorist) – focuses on the result of gaining weight, not the ongoing process: to put on (some) weight as a change of state.

If you said να παίρνω κιλά (imperfective), it would sound more like:

I don’t want to be gaining weight (all the time / generally).

That is possible Greek, but να πάρω κιλά is the more usual idiomatic way to say to put on weight, to gain weight as a result.

What does παίρνω κιλά / να πάρω κιλά literally mean?

Literally:

παίρνω = I take
κιλά = kilos (plural of κιλό)

The phrase παίρνω κιλά is an idiom meaning:

to gain weight, to put on weight

So δεν θέλω να πάρω κιλά = I don’t want to put on weight.

English doesn’t say take kilos, but Greek does.

Why is κιλά plural here? Could we say κιλό?

το κιλό = kilo (singular)
τα κιλά = kilos (plural)

When referring to someone’s body weight, Greek almost always uses the plural:

Έχω πάρει κιλά. = I’ve put on weight.
Έχασε κιλά. = He/She lost weight.

Saying να πάρω κιλό would sound like “to gain one kilo” (a specific amount), which is unusual in a general statement like this. So κιλά is the natural, idiomatic choice.

Why do we use γιατί here, and can it also mean “why”?

γιατί has two main uses:

As “why” (question):
- Γιατί τρως λιγότερα γλυκά; = Why are you eating fewer sweets?
As “because” (connector):
- Τρώω λιγότερα γλυκά, γιατί δεν θέλω να πάρω κιλά.
  = I eat fewer sweets because I don’t want to put on weight.

In your sentence, γιατί means “because”.

You could also use επειδή instead of γιατί here; it would still mean because, with very similar meaning:

… επειδή δεν θέλω να πάρω κιλά.

Could I say για να μην πάρω κιλά instead of γιατί δεν θέλω να πάρω κιλά?

Yes, but the meaning shifts slightly:

… γιατί δεν θέλω να πάρω κιλά.
= … because I don’t want to put on weight. (gives the reason)
… για να μην πάρω κιλά.
= … (in order) not to put on weight. (expresses purpose / goal)

Both are natural:

Θέλω να τρώω λιγότερα γλυκά, γιατί δεν θέλω να πάρω κιλά.
(Reason: I don’t want weight gain.)
Θέλω να τρώω λιγότερα γλυκά, για να μην πάρω κιλά.
(Purpose: I eat fewer sweets so that I won’t gain weight.)

Why is the negation δεν placed before θέλω and not before πάρω?

Negation δεν normally comes right before the conjugated verb of the clause:

δεν θέλω = I do not want

The whole να πάρω κιλά phrase is the object of θέλω. So the sentence structure is:

δεν θέλω [να πάρω κιλά]

= I do not want [to gain weight].

You can also hear:

Θέλω να μην πάρω κιλά. = I want not to gain weight.

This is possible, but there is a nuance:

δεν θέλω να πάρω κιλά = I don’t want to gain weight (the wanting itself is negated).
θέλω να μην πάρω κιλά = I want not to gain weight (slightly stronger focus on the prevention of the weight gain).

Is the repetition of θέλω necessary? Could it be shorter?

The original is:

Θέλω να τρώω λιγότερα γλυκά, γιατί δεν θέλω να πάρω κιλά.

You could, in everyday speech, shorten it slightly to:

Θέλω να τρώω λιγότερα γλυκά, γιατί δεν θέλω να πάρω.
(dropping κιλά, if context is clear)

But you can’t normally drop the second θέλω while keeping γιατί, because γιατί introduces a full clause (with its own verb):

✗ Θέλω να τρώω λιγότερα γλυκά, γιατί να πάρω κιλά. (ungrammatical)

If you want a single θέλω, you’d usually change the connector:

Θέλω να τρώω λιγότερα γλυκά για να μην πάρω κιλά.
(purpose clause, one finite verb θέλω)

Is Θέλω here more like “I want” or “I would like”? How would I make it more polite?

Θέλω by itself is neutral and direct: I want.

To make it softer / more polite, Greek often uses θα ήθελα (I would like):

Θα ήθελα να τρώω λιγότερα γλυκά, γιατί δεν θα ήθελα να πάρω κιλά.
= I would like to eat fewer sweets, because I wouldn’t like to put on weight.

So:

Θέλω… – simple, straightforward
Θα ήθελα… – more polite / tentative / softer in tone