Learn Hardcore Greek: Αντί για τσάι πίνω μόνο νερό το βράδυ. - Instead of tea I drink only water in the evening.

αντί by itself means instead (of), in place of.
In everyday modern Greek, when it is followed by a noun, it usually appears as αντί για + accusative:
- Αντί για τσάι πίνω νερό = Instead of tea, I drink water.

You use αντί για when you want to say that one thing is being replaced by another:
αντί για Χ, κάνω/παίρνω/πίνω Υ = instead of X, I do/take/drink Y.

(With a verb after it, you normally use αντί να + verb: αντί να πίνω τσάι, πίνω νερό = instead of drinking tea, I drink water.)

Why is there no article before τσάι? Could we say το τσάι?

In this sentence τσάι behaves like an uncountable/mass noun: tea in general, not some specific tea. In Greek, with mass nouns used as objects, it is common to omit the article when you mean “some, any, in general”:

Πίνω τσάι. = I drink tea (as a drink, in general).

If you add the article, you usually refer to a specific instance or a clearly defined “the tea”:

Πίνω το τσάι. = I am drinking the tea (the one we have here / that we mentioned).

So:

Αντί για τσάι πίνω μόνο νερό το βράδυ.
= Instead of (drinking) tea, I drink only water in the evening (tea in general).
Αντί για το τσάι πίνω μόνο νερό το βράδυ.
sounds like Instead of the tea (that we were expecting / that I used to drink), I drink only water in the evening.

Both are grammatically correct, but without the article is the normal, neutral choice here.

What case is τσάι in, and why?

Τσάι is in the accusative case, because it is the object of the prepositional phrase αντί για τσάι.

In modern Greek, most prepositions (με, για, σε, από, προς, etc.) are followed by the accusative.
With αντί για, the noun also goes in accusative: αντί για καφέ, αντί για ζάχαρη, αντί για τσάι.

Because τσάι is a neuter noun, its nominative and accusative forms are identical, so you do not see a visible change, but functionally it is accusative.

Can we leave out για and just say Αντί τσάι?

No, αντί τσάι is not correct in standard modern Greek.

You have two correct patterns:

Everyday/neutral: αντί για + accusative
- αντί για τσάι
- αντί για καφέ
More formal: αντί + genitive
- αντί τσαγιού
- αντί καφέ or αντί του καφέ

So your choices are:

Αντί για τσάι πίνω μόνο νερό. ✅
Αντί τσαγιού πίνω μόνο νερό. (formal / written style) ✅

But Αντί τσάι ❌ is not used.

What is the difference between αντί για τσάι and αντί τσαγιού?

They mean essentially the same thing (instead of tea), but they differ in style and grammar:

αντί για τσάι
- Very common in everyday spoken and written modern Greek.
- Uses αντί για + accusative.
- Neutral, natural choice in most contexts.
αντί τσαγιού
- More formal, feels a bit written, “correct”, or old-fashioned.
- Uses αντί + genitive (the more traditional pattern of αντί).

So in normal conversation you would almost always say αντί για τσάι.

Why is the present tense πίνω used? Does it mean “I am drinking now” or “I usually drink”?

Greek present tense πίνω covers both:

I am drinking (right now)
I drink / I usually drink (habit, general truth)

In this sentence, the time expression το βράδυ suggests a habitual meaning:

Αντί για τσάι πίνω μόνο νερό το βράδυ.
= In the evenings, instead of tea, I (habitually) drink only water.

If you want to emphasize that this is a new habit or a change, you might add other words (e.g. πλέον, τώρα) or different constructions, but πίνω itself already naturally expresses a general habit here.

What exactly does μόνο modify here? Can we move it, and does the meaning change?

In πίνω μόνο νερό το βράδυ, μόνο modifies νερό:

μόνο νερό = only water
→ I drink water and nothing else (no tea, no other drinks) in the evening.

In Greek, μόνο is usually placed immediately before the element it limits. Changing its position can change the focus:

Πίνω μόνο νερό το βράδυ.
- Only water (no other drinks) in the evening.
Πίνω νερό μόνο το βράδυ.
- I drink water only in the evening (not in the morning, not at noon).
Μόνο νερό πίνω το βράδυ.
- Same basic meaning as (1), but with stronger emphasis on μόνο νερό (spoken stress).

So position matters: where you put μόνο tells the listener what is being restricted — the drink, the time, or something else.

Why do we say το βράδυ with the article το? Could we also say just βράδυ?

Expressions for times of day very often appear with the neuter article το when they mean “in the …” in a general, habitual way:

το πρωί = in the morning
το μεσημέρι = at noon / in the midday
το βράδυ = in the evening

So:

Πίνω μόνο νερό το βράδυ.
= I drink only water in the evening (in the evenings, as a general habit).

You can find cases without the article, e.g. in short answers or more colloquial/poetic style:

Βράδυ δουλεύεις; = Do you work in the evenings / at night?

But for a neutral sentence like this, το βράδυ is the standard, most natural option.

Can το βράδυ go at the beginning or somewhere else in the sentence?

Yes. Greek word order is quite flexible, especially with time expressions. All of these are possible:

Το βράδυ αντί για τσάι πίνω μόνο νερό.
Το βράδυ πίνω μόνο νερό αντί για τσάι.

Putting το βράδυ at the beginning often adds a bit of emphasis or sets the scene:

Το βράδυ, αντί για τσάι, πίνω μόνο νερό.
→ As for the evening / In the evenings, instead of tea, I drink only water.

The basic meaning stays the same; the difference is mostly in rhythm and emphasis.

Do we need a comma after Αντί για τσάι?

In writing, many speakers would use a comma after a fronted prepositional phrase like this, because it functions a bit like an introductory clause:

Αντί για τσάι, πίνω μόνο νερό το βράδυ. ✅

Without the comma is also grammatically acceptable:

Αντί για τσάι πίνω μόνο νερό το βράδυ. ✅

So the comma is optional; it mainly affects clarity and rhythm in written text, not the grammar.

Can we also say Πίνω μόνο νερό το βράδυ αντί για τσάι? Is that the same?

Yes, that sentence is perfectly correct and means the same thing overall:

Πίνω μόνο νερό το βράδυ αντί για τσάι. ✅

Here the core statement Πίνω μόνο νερό το βράδυ comes first, and αντί για τσάι is added at the end, specifying “instead of tea”.

Compared with Αντί για τσάι πίνω μόνο νερό το βράδυ, the difference is just in emphasis:

Fronted Αντί για τσάι highlights the replacement more strongly.
Final …αντί για τσάι states what you do first, and then adds “instead of tea” as extra information.

In everyday speech, both word orders are natural.

Could we say πίνω μόνο το νερό instead of πίνω μόνο νερό? What would change?

Yes, but the meaning changes:

πίνω μόνο νερό
- νερό without article = water in general, some amount of water.
- Means: I drink only water (as a type of drink, not other drinks).
πίνω μόνο το νερό
- το νερό = the water, something specific that both speaker and listener can identify (for example, the water we ordered, the water on the table).
- Means: I drink only the water (and not something else specific, like the wine, the juice, etc.).

In your original sentence, because we are talking about general drinking habits in the evening, πίνω μόνο νερό (without article) is the natural, general statement.