Βάζω τσάι σε μεγάλη κούπα όταν κάνει κρύο, ενώ η αδερφή μου πίνει καφέ σε μπλε κούπα.

Greek βάζω is very broad. It often means put, but in everyday speech it can also mean things like:

• put/place
• pour/serve
• add

So Βάζω τσάι σε μεγάλη κούπα is a natural way to say that you put or pour tea into a big mug.

Depending on context, English might translate it more naturally as:

• I put tea in a big mug
• I pour tea into a big mug
• I make myself tea in a big mug

Greek often uses one common verb where English prefers a more specific one.

Because τσάι and καφέ here are being used as general, indefinite substances: tea and coffee, not the tea or the coffee.

This is very common with drinks, food, and other mass nouns.

So:

• πίνω καφέ = I drink coffee
• πίνω τον καφέ = I drink the coffee / I’m drinking my coffee in a specific context

In your sentence, the speaker is talking generally about what they drink/use, so no article is needed.

Because καφέ is the accusative singular form of καφές.

• nominative: ο καφές = the coffee
• accusative: τον καφέ = the coffee as an object

In πίνει καφέ, coffee is the direct object of drinks, so Greek uses the accusative.

With τσάι, the nominative and accusative look the same:

• το τσάι
• πίνω τσάι

So you do not see a form change there.

Greek can leave the noun phrase indefinite without explicitly saying a.

So σε μεγάλη κούπα means:

• in a big mug

You could also say:

• σε μια μεγάλη κούπα = in a big mug / in one big mug

Adding μια makes the indefiniteness more explicit, a bit like English a. Without it, the phrase is still perfectly natural.

If the mug were definite, Greek would use the article:

• στη μεγάλη κούπα = in the big mug

Because the adjective must agree with the noun in gender, number, and case.

κούπα is:

• feminine
• singular

So the adjective must also be feminine singular:

• μεγάλη κούπα = big mug

Compare:

• μεγάλος καφές = big coffee (masculine)
• μεγάλο ποτήρι = big glass (neuter)
• μεγάλη κούπα = big mug (feminine)

Because μπλε is an indeclinable adjective. That means it usually stays the same regardless of gender, number, or case.

So you get:

• μπλε κούπα
• μπλε ποτήρι
• μπλε καφές would still use μπλε

By contrast, μεγάλος / μεγάλη / μεγάλο changes form to match the noun.

This is common with some color words in modern Greek, especially borrowed ones like μπλε.

This is a very common Greek weather expression.

• κάνει κρύο = it is cold
• literally: it does cold or it makes cold

Greek often uses κάνει in weather expressions:

• κάνει ζέστη = it’s hot
• κάνει κρύο = it’s cold
• κάνει αέρα = it’s windy

So όταν κάνει κρύο simply means when it’s cold.

Here ενώ means something like:

• while
• whereas

It connects two contrasting ideas:

• I put tea in a big mug when it’s cold, whereas my sister drinks coffee in a blue mug.

So in this sentence, ενώ has a mildly contrastive meaning, not just a time meaning.

Depending on context, English could translate it as either:

• while
• whereas

Greek normally uses the definite article with possessives.

So:

• η αδερφή μου = my sister
• literally something like the sister of mine

This is very natural Greek structure. English usually does my sister without the, but Greek generally keeps the article.

You will see this with many family words and other nouns:

• ο πατέρας μου = my father
• η φίλη μου = my friend
• το σπίτι μου = my house

Both exist.

• αδελφή is the more formal/traditional form
• αδερφή is extremely common in everyday modern Greek

So η αδερφή μου is perfectly natural and common in spoken and informal written Greek.

You may also hear:

• αδελφός / αδερφός = brother
• αδελφή / αδερφή = sister

Because Greek usually does not need subject pronouns unless there is emphasis or contrast.

The verb ending already tells you the subject:

• βάζω = I put
• πίνει = he/she drinks

So Βάζω τσάι... already means I put tea...

If you said Εγώ βάζω τσάι..., it would sound more emphatic, like:

• I put tea...
• As for me, I put tea...

Greek drops subject pronouns much more often than English does.

It sounds habitual or generally true, not like a one-time event.

The present tense here suggests something like:

• I put tea in a big mug when it’s cold, while my sister drinks coffee in a blue mug.

So it describes a usual pattern or preference.

Greek present tense often works like English simple present for habits:

• πίνω καφέ το πρωί = I drink coffee in the morning
• βάζω τσάι όταν κάνει κρύο = I put/make tea when it’s cold

After σε, modern Greek uses the accusative.

So in principle:

• σε + accusative

In this sentence:

• σε μεγάλη κούπα
• σε μπλε κούπα

Both noun phrases are accusative.

However, with a feminine noun like κούπα, the nominative and accusative singular look the same, so you do not see an obvious form change.

You can see the case more clearly if there is a definite article:

• η κούπα = nominative
• την κούπα = accusative
• στην κούπα / στην κούπα or commonly στην κούπα, often shortened in speech depending on context = in the mug

In everyday spelling, with σε + την, you normally get στην.

Yes. Greek word order is more flexible than English word order.

The original sentence is natural:

• Βάζω τσάι σε μεγάλη κούπα όταν κάνει κρύο, ενώ η αδερφή μου πίνει καφέ σε μπλε κούπα.

But Greek could move things around for emphasis, for example:

• Όταν κάνει κρύο, βάζω τσάι σε μεγάλη κούπα...
• Η αδερφή μου πίνει καφέ σε μπλε κούπα, ενώ εγώ βάζω τσάι...

The meaning stays mostly the same, but the emphasis changes.

Greek often uses word order to highlight what is most important or contrastive.