Χτες γνώρισα τον αδερφό της και μου έκανε πολύ καλή εντύπωση.

Yes. Χτες means yesterday. It is a very common everyday spelling and pronunciation of χθες.

• χθες is the more traditional spelling
• χτες is very common in modern informal writing and speech

Both mean the same thing.

Γνώρισα is the aorist form, which is used for a completed event in the past: I met / I got to know.

So in this sentence, Χτες γνώρισα... means:

• Yesterday I met...

By contrast, γνώριζα is the imperfect, which usually means something ongoing, repeated, or descriptive in the past:

• I was knowing / I knew / I used to know

So γνώρισα is the natural choice for a single completed event: meeting someone yesterday.

Greek often leaves out subject pronouns because the verb ending already shows who the subject is.

Here:

• γνώρισα = I met
• the ending -α tells you the subject is I

So Greek does not need εγώ unless you want emphasis:

• Εγώ γνώρισα... = I met... with extra emphasis

Because Greek usually uses the definite article where English often does too, and sometimes even where English would not emphasize it as much.

• τον = the (masculine accusative singular)
• αδερφό = brother

So τον αδερφό means the brother.

In this sentence, γνώρισα τον αδερφό της means I met her brother.

It is in the accusative case because it is the direct object of γνώρισα.

You met whom?

• τον αδερφό της

Masculine singular forms here are:

• nominative: ο αδερφός
• accusative: τον αδερφό

So after a verb like γνώρισα (I met), Greek uses the accusative.

Here της means her.

In this sentence:

• τον αδερφό της = her brother

Grammatically, της is a genitive pronoun showing possession.

So Greek literally structures this as something like:

• the brother of her

But in natural English, we say:

• her brother

Yes. Both exist.

• αδερφός / αδερφό is very common in everyday modern Greek
• αδελφός / αδελφό is the more formal or conservative form

They mean the same thing: brother.

So learners should recognize both spellings.

Literally, it means something like:

• μου = to me
• έκανε = made
• πολύ καλή εντύπωση = a very good impression

So the whole phrase is literally:

• he made a very good impression on me

In natural English, that is usually:

• I was very impressed by him
• He made a very good impression on me

Μου is the weak form of σε μένα / to me.

In the expression μου έκανε εντύπωση, it marks the person affected:

• μου έκανε εντύπωση = it impressed me / it made an impression on me

So:

• μου = to me
• σου = to you
• του/της = to him/her

Examples:

• Μου έκανε εντύπωση. = It impressed me.
• Σου έκανε εντύπωση; = Did it impress you?

Because κάνω εντύπωση is a very common Greek idiom.

It literally means make an impression, but it is often the normal way to express:

• impress
• strike someone
• leave an impression

So μου έκανε πολύ καλή εντύπωση is a very natural Greek way to say:

• He made a very good impression on me
• I was very favorably impressed

Πολύ καλή means very good and it describes εντύπωση.

• εντύπωση = impression
• καλή εντύπωση = good impression
• πολύ καλή εντύπωση = very good impression

Notice that:

• καλή is feminine singular
• it agrees with εντύπωση, which is also feminine singular

The subject is understood to be the same person mentioned earlier: τον αδερφό της.

So the sentence means:

• Yesterday I met her brother, and he made a very good impression on me.

Greek often does not repeat the subject if it is clear from context.

So after και, Greek does not need to say αυτός (he). It is simply understood.

In theory, Greek can sometimes leave things a bit open, but in this sentence the most natural interpretation is that her brother made the good impression.

So the understood meaning is:

• Yesterday I met her brother, and he made a very good impression on me.

That is by far the most likely reading.

Και means and.

It links the two parts of the sentence:

• Χτες γνώρισα τον αδερφό της
• και μου έκανε πολύ καλή εντύπωση

So:

• Yesterday I met her brother, and he made a very good impression on me.

It is natural, neutral Greek word order.

Greek word order is more flexible than English, but this sentence sounds very normal as it is:

• Χτες at the beginning sets the time
• γνώρισα τον αδερφό της gives the main event
• και μου έκανε πολύ καλή εντύπωση adds the result/impression

You could move some parts around for emphasis, but this version is straightforward and idiomatic.

A rough pronunciation for an English speaker is:

• HTES GNO-ri-sa ton a-der-FO tis kee moo E-ka-ne po-LEE ka-LEE en-DEE-po-see

A few helpful notes:

• χ in Χτες is not an English h exactly; it is a harsher sound
• γν in γνώρισα is pronounced together, not as two separate full syllables
• της sounds like tees
• εντύπωση is stressed on τύ: en-DI-po-si

It is neutral everyday modern Greek.

A few details make it sound conversational and natural:

• Χτες instead of the more formal-looking χθες
• αδερφό instead of the more formal αδελφό

But overall, the sentence is perfectly standard and natural in normal speech and writing.