Βάζω τσάι σε μεγάλη κούπα όταν κάνει κρύο, αλλά ο αδερφός μου προτιμά καφέ.

βάζω is the 1st person singular, present tense form of the verb βάζω.

So it means I put / I place / I pour depending on context.

The ending -ω is a very common present-tense ending for I in Greek.
Because the verb ending already shows the subject, Greek does not need a separate word for I here.

So:

• βάζω = I put
• not necessarily εγώ βάζω, unless you want extra emphasis on I

Yes, βάζω literally means put, place, or set, but in everyday Greek it is also used in very natural ways where English might say:

• pour
• make
• serve
• put into

In Βάζω τσάι σε μεγάλη κούπα, the idea is something like:

• I put/pour tea into a big mug

So this is a normal Greek way to describe preparing or serving tea into a mug.

Greek often leaves out the article when talking about a substance or something in a general, non-specific way.

Here τσάι means tea in general, not the tea or a specific tea.

So:

• βάζω τσάι = I put/pour tea
• βάζω το τσάι = I put/pour the tea (a specific tea already known in context)

This is similar to English mass nouns:

• I drink tea
• not usually I drink a tea unless you mean one serving

All three are possible, but they mean slightly different things.

• σε μεγάλη κούπα = in a big mug / into a big mug, in a general sense
• σε μια μεγάλη κούπα = in a big mug, with extra emphasis on a
• στη μεγάλη κούπα = in the big mug, a specific mug already known

In your sentence, σε μεγάλη κούπα sounds natural because it describes the kind of container used, not one particular mug.

So the speaker is talking generally about their habit.

μεγάλη is the adjective big, and it agrees with κούπα.

κούπα is:

• feminine
• singular
• here in the accusative after σε

So the adjective matches it:

• μεγάλη κούπα = big mug

This is a basic Greek rule: adjectives agree with the noun in gender, number, and case.

κούπα is in the accusative singular.

That is because it comes after the preposition σε, which in Modern Greek normally takes the accusative.

So:

• σε κούπα
• σε μεγάλη κούπα

Even though the form κούπα looks the same as the nominative here, its function is accusative because of σε.

Literally, κάνει κρύο means it makes cold, but idiomatically it means:

• it is cold
• the weather is cold

This is a very common Greek weather expression.

Examples:

• κάνει ζέστη = it is hot
• κάνει κρύο = it is cold
• κάνει καλό καιρό = the weather is good

So όταν κάνει κρύο means when it’s cold.

Greek often does not use a dummy subject like English it in weather expressions.

English says:

• when it is cold

Greek naturally says:

• όταν κάνει κρύο

There is no need for a separate word equivalent to English weather it.

That is normal Greek structure.

Greek usually uses the definite article with family members and possessive expressions.

So Greek says:

• ο αδερφός μου = literally the brother my
• natural English: my brother

This is completely normal Greek grammar.

Other examples:

• η μητέρα μου = my mother
• ο φίλος μου = my friend
• το σπίτι μου = my house

So even though English does not use the here, Greek usually does.

μου is a weak possessive pronoun meaning my, and in Greek these short possessive forms usually come after the noun.

So:

• ο αδερφός μου = my brother
• η αδερφή μου = my sister
• το βιβλίο μου = my book

This is the normal position in Greek.

Both exist.

• αδελφός is the more traditional and historically older form
• αδερφός is a very common modern everyday form

In ordinary spoken Modern Greek, αδερφός is extremely common and natural.

So learners should recognize both, but αδερφός is not wrong at all.

προτιμά is the 3rd person singular, present tense of προτιμώ = prefer.

So:

• προτιμώ = I prefer
• προτιμάς = you prefer
• προτιμά = he/she/it prefers

Here the subject is ο αδερφός μου, so Greek uses the he prefers form:

• ο αδερφός μου προτιμά καφέ = my brother prefers coffee

For the same reason as with τσάι: it is being used in a general sense.

• προτιμά καφέ = he prefers coffee
• προτιμά τον καφέ can also occur, but usually sounds more specific or more like coffee as a category in a particular contrast

In many everyday sentences, when talking about food or drink in general, Greek may omit the article.

So here καφέ means coffee in general, not a particular coffee.

The dictionary form is ο καφές = coffee / coffee drink.

But here it is the direct object of προτιμά, so it is in the accusative singular:

• nominative: ο καφές
• accusative: τον καφέ or simply καφέ

So:

• ο καφές είναι ζεστός = the coffee is hot
• προτιμά καφέ = he prefers coffee

That final -ς often disappears in the accusative singular of many masculine nouns.

αλλά means but.

It connects two contrasting ideas:

• I put/pour tea into a big mug when it’s cold
• but my brother prefers coffee

So it introduces a contrast between the speaker’s preference and the brother’s preference.

Greek word order is more flexible than English, although this sentence is already very natural.

Current order:

• Βάζω τσάι σε μεγάλη κούπα όταν κάνει κρύο, αλλά ο αδερφός μου προτιμά καφέ.

This is neutral and normal.

Greek can move elements around for emphasis, for example:

• Όταν κάνει κρύο, βάζω τσάι σε μεγάλη κούπα.
• Καφέ προτιμά ο αδερφός μου.
  This gives extra emphasis to coffee.

So the exact order can change, but the original sentence is a standard, natural way to say it.

In this sentence, it most naturally suggests a habitual action:

• When it’s cold, I put/pour tea into a big mug

The present tense in Greek often works this way for regular habits or general truths.

So the sentence sounds like something the speaker usually does whenever the weather is cold.