Βάζω νερό στην κούπα μου και όχι στο ποτήρι.

Because these are contractions of σε + the definite article:

• στην = σε + την
• στο = σε + το

The article changes because the nouns have different genders:

• η κούπα = feminine
• το ποτήρι = neuter

So:

• στην κούπα = into/in the cup
• στο ποτήρι = into/in the glass

Greek uses these contracted forms constantly in everyday speech and writing.

Because νερό here is being used as an uncountable substance, like water in English.

In English, we usually say:

• I put water in my cup not
• I put the water in my cup unless we mean some specific water

Greek works similarly here. Βάζω νερό means I put water / I pour water in a general, indefinite sense.

If you said το νερό, that would usually mean the water, referring to specific water already known from context.

In Greek, possessive pronouns like μου often come after the noun:

• η κούπα μου = my cup
• το βιβλίο μου = my book
• ο φίλος μου = my friend

So κούπα μου is the normal Greek word order.

This is very different from English, where my comes before the noun.

Μου means my here.

More literally, it comes from the weak form of I / me, but in this structure it functions as a possessive:

• η κούπα μου = my cup
• το σπίτι μου = my house

So in this sentence, στην κούπα μου means in/into my cup.

Greek σε is very flexible. It can mean:

• in
• on
• to
• into

The exact meaning depends on the verb and the context.

With a verb like βάζω (put), σε often corresponds to English into or in:

• βάζω νερό στην κούπα = I put/pour water into the cup

So even though Greek uses σε, English may translate it as in or into depending on what sounds natural.

They are in the accusative case, because σε takes the accusative.

However, in these nouns, the accusative looks the same as the nominative:

• η κούπα → την κούπα
• το ποτήρι → το ποτήρι

So the form of the noun itself does not visibly change here.

The article is what shows the case most clearly:

• η κούπα → την κούπα
• το ποτήρι → το ποτήρι (same form)

This is very common in Modern Greek, especially with neuter nouns, where nominative and accusative are often identical.

Βάζω is a very common verb meaning things like:

• put
• place
• set
• sometimes pour in the right context

In this sentence, because the object is water, English naturally understands it as put/pour water.

So βάζω is broader than just one English verb. Greek often uses it in situations where English might choose different verbs depending on the object.

For example:

• βάζω το βιβλίο στο τραπέζι = I put the book on the table
• βάζω νερό στην κούπα = I put/pour water into the cup

Because όχι is used to negate the phrase στο ποτήρι as a contrast:

• και όχι στο ποτήρι = and not in the glass

Here, the speaker is not negating the whole verb. They are correcting or contrasting the destination.

Compare:

• Δεν βάζω νερό στην κούπα μου. = I am not putting water in my cup.
• Βάζω νερό στην κούπα μου και όχι στο ποτήρι. = I put water in my cup and not in the glass.

So:

• δεν negates the verb/clause
• όχι negates or contrasts a word or phrase

Yes, that is possible.

• και όχι στο ποτήρι sounds like and not in the glass
• αλλά όχι στο ποτήρι sounds more like but not in the glass

Both can work, but the nuance is slightly different.

και όχι is very common when adding a correction or clarification:

• στον Γιάννη και όχι στη Μαρία
• today and not tomorrow

αλλά όχι can sound a bit more strongly contrastive.

In your sentence, και όχι sounds perfectly natural.

Greek uses definite articles much more often than English.

Here, στην κούπα μου is natural because my cup is a specific thing, and Greek normally keeps the article:

• στην κούπα μου literally = in the cup my
• natural English = in my cup

Similarly, στο ποτήρι refers to the glass, presumably a specific glass in the situation.

Greek usually says:

• στο σπίτι μου = at my house
• με τον φίλο μου = with my friend
• στη δουλειά μου = at my job/work

So the article is not optional here in normal Greek.

The word order can change, because Greek is more flexible than English.

Your sentence is perfectly natural as:

• Βάζω νερό στην κούπα μου και όχι στο ποτήρι.

But you could also hear variations like:

• Βάζω νερό όχι στο ποτήρι, αλλά στην κούπα μου.
• Στην κούπα μου βάζω νερό, όχι στο ποτήρι.

Changing the order changes the emphasis more than the basic meaning.

The original version is a neutral, natural way to say it.

No. Greek has several words depending on the type of cup.

For example:

• κούπα = mug, large cup
• φλιτζάνι = cup, especially a smaller cup like a coffee cup

So κούπα suggests something more like a mug or larger handled cup, while φλιτζάνι often suggests a smaller cup.

The choice depends on what kind of container the speaker means.

Because βάζω is in the present tense, it can mean either:

• what you are doing now, depending on context
• a habitual action
• a general statement

So this sentence could mean:

• Right now, I’m putting water in my cup and not in the glass or
• I put water in my cup, not in the glass as a usual preference

Greek present tense often covers both simple present and present continuous, and context tells you which one is meant.