Η γάτα νιαούριζε χτες όλο το βράδυ, αλλά σήμερα είναι ήσυχη.

νιαούριζε is the imperfect tense of νιαουρίζω = to meow.

In this sentence, the imperfect suggests an action that was:

• ongoing
• repeated
• or happening over a stretch of time in the past

So Η γάτα νιαούριζε χτες όλο το βράδυ gives the idea of the cat was meowing all night yesterday, not just one single meow.

It is 3rd person singular, so it matches η γάτα = the cat.

This is a very common distinction in Greek.

• νιαούριζε = was meowing / kept meowing
• νιαούρισε = meowed (more like a single completed event, or a summary of it)

Because the sentence includes όλο το βράδυ = all night, the imperfect fits very naturally: it presents the meowing as something that continued through the night.

So:

• Η γάτα νιαούριζε όλο το βράδυ = The cat was meowing all night.
• Η γάτα νιαούρισε = The cat meowed.

The imperfect is the better choice when English would often use was ...-ing.

Yes. χτες and χθες both mean yesterday.

A learner should know this:

• χθες is often seen as the more formal or standard spelling
• χτες is extremely common in everyday speech and writing

So in this sentence, χτες is completely normal and natural.

όλο το βράδυ means all night.

Word by word:

• όλο = whole / all
• το βράδυ = the night / the evening, depending on context

Together, όλο το βράδυ means the whole night or more naturally in English, all night.

This is a very common Greek pattern:

• όλη τη μέρα = all day
• όλο το πρωί = all morning
• όλη την εβδομάδα = all week

So the article is normal here. Greek often says the whole X, where English may just say all X.

Because Greek normally uses the definite article in expressions like the whole night, the whole day, the whole week.

So:

• όλο το βράδυ literally = the whole night
• natural English translation = all night

This is just the standard Greek structure:

• όλος / όλη / όλο
  • article
    • noun

The form όλο is used because βράδυ is:

• neuter
• singular

Compare:

• όλη τη νύχτα = all night
• όλο το βράδυ = all night

Both are possible, but with different nouns.

Η γάτα is nominative singular feminine.

That is because it is the subject of the sentence:

• The cat was meowing...
• ...but today it is quiet.

The article η is the feminine singular nominative form of the, and γάτα is a feminine noun.

So:

• η γάτα = the cat
• μια γάτα = a cat

Because it describes η γάτα, which is a feminine singular noun.

Greek adjectives agree with the noun they describe in:

• gender
• number
• case

So here:

• η γάτα = feminine singular
• therefore ήσυχη must also be feminine singular

Compare:

• ο σκύλος είναι ήσυχος = the dog is quiet
• η γάτα είναι ήσυχη = the cat is quiet
• το παιδί είναι ήσυχο = the child is quiet

Because the sentence is contrasting yesterday with today.

• χτες refers to the past, so Greek uses a past tense: νιαούριζε
• σήμερα refers to the present, so Greek uses the present tense: είναι

So the sentence creates a clear contrast:

• yesterday: ongoing meowing
• today: present state of quietness

This is very natural Greek, just like in English:

• The cat was meowing all night yesterday, but today it is quiet.

Because Greek usually does not need subject pronouns when the subject is already clear.

The verb ending and the context usually tell you who or what the subject is.

So instead of saying something like:

• ...but today it is quiet

Greek simply says:

• αλλά σήμερα είναι ήσυχη

The subject is understood to still be η γάτα.

Greek can include pronouns for emphasis, but it normally leaves them out when they are not needed.

Greek word order is more flexible than English word order.

The version in your sentence is natural:

• Η γάτα νιαούριζε χτες όλο το βράδυ

But other orders are also possible, depending on emphasis:

• Χτες η γάτα νιαούριζε όλο το βράδυ
• Η γάτα χτες νιαούριζε όλο το βράδυ
• Όλο το βράδυ χτες η γάτα νιαούριζε (less neutral, more marked)

The original sentence sounds very natural and conversational. The main idea is that Greek can move time expressions around more freely than English.

αλλά means but.

It introduces a contrast:

• yesterday the cat was noisy
• today the cat is quiet

Yes, the comma is normal. It separates the two clauses:

• Η γάτα νιαούριζε χτες όλο το βράδυ
• αλλά σήμερα είναι ήσυχη

That punctuation works very much like English.

A rough pronunciation guide is:

i GHA-ta nia-OO-ri-ze htes O-lo to VRA-thi, a-LA SI-me-ra EE-ne EE-si-hi

A few useful notes:

• η sounds like ee
• γ before α is a soft sound, somewhat like a voiced throaty gh
• νια in νιαούριζε sounds like nya
• χ in χτες is like the ch in German Bach or Scottish loch
• βράδυ is stressed on the first syllable: VRA-
• σήμερα is stressed on SI-
• ήσυχη is stressed on the first syllable: EE-

You do not need a perfect English equivalent for every sound right away; the most important thing at first is to keep the stress in the right place.

Yes, absolutely.

That would also mean The cat was meowing all night.

Compare:

• όλο το βράδυ = all night / the whole evening-night period
• όλη τη νύχτα = all night, using night more directly

Both are natural. The choice depends on which noun you want:

• βράδυ = evening / night, depending on context
• νύχτα = night

In everyday Greek, both can work well in this kind of sentence.