Learn Hardcore Greek: Βάζω λίγη ζάχαρη στον καφέ μου.

So Βάζω λίγη ζάχαρη στον καφέ μου. already means I put a little sugar in my coffee without needing an extra εγώ (I). You could say Εγώ βάζω… for emphasis (I put…), but it is not required.

What tense or aspect is βάζω? Does it mean I put or I am putting?

Βάζω is the present tense, imperfective aspect. In practice, it covers both:

I put a little sugar in my coffee (habitually, in general)
I am putting a little sugar in my coffee (right now)

Greek does not normally distinguish between simple and continuous the way English does. Context tells you whether it is a general habit or a present action.

Can you explain what each word is grammatically?

Yes, word by word:

Βάζω – verb, present tense, 1st person singular: I put / I am putting
λίγη – adjective of quantity, feminine, singular, accusative: a little (bit of)
ζάχαρη – noun, feminine, singular, accusative: sugar
στον – σε (in / to) + τον (the, masculine accusative) merged: in the / into the
καφέ – noun, masculine, singular, accusative: coffee
μου – unstressed possessive pronoun: my

So literally: I-put little sugar in-the coffee my.

Why is it λίγη ζάχαρη and not λίγο ζάχαρη?

The basic rule is that adjectives agree with the noun in gender, number, and case.

ζάχαρη is feminine singular accusative.
The adjective λίγος (little/few) in feminine singular accusative is λίγη.
So: λίγη ζάχαρη = a little sugar.

You will also hear βάζω λίγο ζάχαρη, where λίγο is used adverbially, more like a bit of sugar. Both forms are common and acceptable:

βάζω λίγη ζάχαρη – more literally I put a small amount of sugar
βάζω λίγο ζάχαρη – I put a bit of sugar

For learning the agreement pattern, λίγη ζάχαρη is a very clear example.

Why is there no article with ζάχαρη? Why not τη ζάχαρη?

Mass nouns that refer to an indefinite amount usually appear without an article in Greek, just like in English:

βάζω ζάχαρη – I put sugar
πίνω νερό – I drink water

You would use τη ζάχαρη (the sugar) when you mean some specific sugar that both speakers know about, for example:

Βάλε τη ζάχαρη στο ντουλάπι. – Put the sugar in the cupboard.

In your sentence, we are talking about sugar in general, so no article: λίγη ζάχαρη.

What is στον exactly? Why not σε τον?

στον is the contracted form of:

σε (a very common preposition: in, at, to, on)
τον (the, masculine, singular, accusative)

So:

σε + τον καφέ → στον καφέ (in the coffee / into the coffee)

Greek normally uses these contractions in speech and writing:

σε + τον → στον
σε + την → στην
σε + το → στο

Writing σε τον καφέ is grammatically understandable but feels unnatural; στον καφέ is the normal form.

Why is it καφέ here and not καφές?

Καφές is a masculine noun. Its main singular forms are:

ο καφές – the coffee (nominative, subject)
του καφέ – of the coffee (genitive)
τον καφέ – the coffee (accusative, object)

In your sentence, καφέ is the object of the preposition σε (into what? into the coffee), so it must be in the accusative:

στον καφέ μου = σε + τον καφέ μου = into my coffee

That is why you see καφέ and not καφές.

What does μου do, and why does it come after καφέ?

Μου here is an unstressed possessive pronoun meaning my.

In Greek, these short possessive pronouns usually come after the noun (and after the article), not before it like in English:

ο καφές μου – my coffee
το βιβλίο σου – your book
το σπίτι του – his house

So:

στον καφέ μου = in my coffee

There is another form with δικός that is used for emphasis, for example ο δικός μου καφές (my own coffee), but the simple ο καφές μου is the normal way to say my coffee.

Can the word order change, or must it be Βάζω λίγη ζάχαρη στον καφέ μου?

Greek word order is more flexible than English, because grammar is marked by endings rather than position. You might hear:

Λίγη ζάχαρη βάζω στον καφέ μου. – I put a little sugar in my coffee.
Στον καφέ μου βάζω λίγη ζάχαρη. – In my coffee I put a little sugar (maybe contrasting with someone else’s coffee).

The neutral, most common order in everyday speech is exactly what you have:

Βάζω λίγη ζάχαρη στον καφέ μου.

Changing the order usually adds emphasis or contrast, not a different basic meaning.

How would I say I don’t put sugar in my coffee?

To make it negative, you add δεν (or δε before some consonants) before the verb:

Δεν βάζω ζάχαρη στον καφέ μου.
= I don’t put sugar in my coffee.

If you want to keep the idea of a little, you could say:

Δεν βάζω καθόλου ζάχαρη στον καφέ μου. – I don’t put any sugar in my coffee at all.

How would the sentence change with different persons, like you or we?

Only the verb and the possessive pronoun need to change. For example:

Βάζεις λίγη ζάχαρη στον καφέ σου. – You (singular) put a little sugar in your coffee.
Βάζει λίγη ζάχαρη στον καφέ του / της. – He / She puts a little sugar in his / her coffee.
Βάζουμε λίγη ζάχαρη στον καφέ μας. – We put a little sugar in our coffee.
Βάζετε λίγη ζάχαρη στον καφέ σας. – You (plural / polite) put a little sugar in your coffee.
Βάζουν(ε) λίγη ζάχαρη στον καφέ τους. – They put a little sugar in their coffee.

Note how μου, σου, του, της, μας, σας, τους change with the person.

How do you pronounce this sentence, and are there any tricky sounds for English speakers?

A rough phonetic guide (using English-like notation) is:

Βάζω – VA-zo (β is like English v, not b)
λίγη – LEE-yi (the γ before η is a soft y-like sound, somewhat like the y in you but fricative)
ζάχαρη – ZA-ha-ree (χ is a rough h, like the ch in German Bach)
στον – ston (short o, like stop)
καφέ – ka-FE (stress on the last syllable)
μου – moo (like English moo)

Stress marks in Greek (the accent) show which syllable is stressed:
ΒΆζω ΛΊγη ΖΆχαρη στον καΦΈ μου.
Stressing the correct syllables is important for sounding natural.