Η αδερφή μου πίνει τσάι σε μεγάλη κούπα το βράδυ.

Η is the definite article for a feminine singular noun in the nominative case, so it means the.

• η αδερφή = the sister
• Η αδερφή μου = my sister

In Greek, when you say my sister, you normally still use the article, unlike in English. So Greek says something closer to the sister of me.

μου means my, but grammatically it is really a weak genitive pronoun meaning of me.

So:

• η αδερφή μου = my sister
• literally: the sister of me

In Greek, these short possessive forms usually come after the noun:

• η μητέρα μου = my mother
• το σπίτι μου = my house

That is completely normal Greek word order.

πίνει is the 3rd person singular form of the verb πίνω = to drink.

So:

• πίνω = I drink
• πίνεις = you drink
• πίνει = he/she/it drinks

Because the subject is Η αδερφή μου = my sister, Greek uses πίνει.

Greek usually does not need a separate subject pronoun like she, because the verb ending already shows the person and number.

Greek often leaves out subject pronouns when they are not needed.

So instead of saying:

• Η αδερφή μου αυτή πίνει... or using she

Greek simply says:

• Η αδερφή μου πίνει...

The subject is already clear from η αδερφή μου and from the verb form πίνει.

Greek often omits the article with mass nouns or when talking about something in a general, indefinite way.

So:

• πίνει τσάι = she drinks tea

This sounds natural when you mean the drink itself, not a specific tea.

If you said πίνει το τσάι, that would usually mean she is drinking the tea — a particular tea that is already known in the context.

The correct form here is μεγάλη because the adjective must agree with κούπα.

κούπα is:

• feminine
• singular

So the adjective must also be feminine singular:

• μεγάλη κούπα = large mug

Greek adjectives change form depending on the noun’s:

• gender
• number
• case

Here both words match.

Greek can use σε + noun without an article when the meaning is in a large mug or into a large mug in a more general sense.

So:

• σε μεγάλη κούπα = in a large mug

You could also hear:

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

Adding μια makes it sound a bit more explicitly indefinite, like a in English. Without it, the phrase is still very natural.

σε usually means in, into, at, to, or on, depending on context.

In this sentence:

• σε μεγάλη κούπα means something like in a large mug or from a large mug, depending on how you interpret the situation.

Greek σε is very common and covers several ideas that English expresses with different prepositions.

το βράδυ is a very common Greek time expression.

It literally means the evening or the night-time, but in context it is often translated as:

• in the evening
• at night

Greek often uses the article in time expressions:

• το πρωί = in the morning
• το μεσημέρι = at noon / in the midday period
• το απόγευμα = in the afternoon
• το βράδυ = in the evening / at night

So the article here is normal and does not mean English the in a strong way.

Greek word order is fairly flexible, but the end position is very natural for a time phrase.

So this sentence is structured like:

• subject: Η αδερφή μου
• verb: πίνει
• object: τσάι
• additional phrase: σε μεγάλη κούπα
• time: το βράδυ

Putting το βράδυ at the end sounds natural and clear. Greek often places time expressions at the beginning or end:

• Το βράδυ η αδερφή μου πίνει τσάι...
• Η αδερφή μου πίνει τσάι... το βράδυ

Both are possible.

Yes. Greek word order is more flexible than English, because case endings and verb forms help show the relationships between words.

For example, you could also say:

• Το βράδυ η αδερφή μου πίνει τσάι σε μεγάλη κούπα.
• Η αδερφή μου το βράδυ πίνει τσάι σε μεγάλη κούπα.

These all mean basically the same thing, but the emphasis changes slightly.

The original sentence is a neutral, natural order.

No. Greek also has αδελφή.

Both exist, but:

• αδερφή is very common in everyday modern speech
• αδελφή is more formal or traditional in some contexts

A learner should definitely recognize both. In ordinary conversation, αδερφή is extremely common.

The accent marks show which syllable is stressed.

• αδερφή → stress on the last syllable
• πίνει → stress on the first syllable
• τσάι → stress on the first part
• μεγάλη → stress on γά
• κούπα → stress on κού
• βράδυ → stress on βρά

Stress matters in Greek pronunciation, so the written accents are important.

In modern Greek, σε is followed by the accusative.

So in:

• σε μεγάλη κούπα

κούπα is in the accusative singular. For many feminine nouns like κούπα, the nominative and accusative forms are the same, so you do not see a change in the ending.

The adjective also matches that case:

• μεγάλη κούπα

A simple breakdown is:

• Η αδερφή μου = subject
• πίνει = verb
• τσάι = direct object
• σε μεγάλη κούπα = prepositional phrase
• το βράδυ = time expression

So the sentence is built very logically:

My sister + drinks + tea + in a large mug + at night / in the evening

In this sentence, τσάι is singular and means tea as a drink or substance.

Like English tea, it is often treated as a mass noun:

• πίνει τσάι = she drinks tea

If you were talking about different kinds of teas, Greek can also use plural forms in the right context, but here it is just the ordinary singular mass-noun use.

It can be either, depending on context.

• βράδυ often covers what English calls evening
• in some contexts it can also be translated night

So το βράδυ is a flexible time expression. If the exact time matters, context will decide the best English translation.