Είδαμε το κάστρο χτες.

Είδαμε means we saw.

Greek verbs usually show the subject through their ending, so a separate subject pronoun is often unnecessary. The ending -αμε tells you the subject is we.

So:

• είδαμε = we saw
• not usually we saw with a separate we

You could add εμείς for emphasis, but normally it is omitted:

• Εμείς είδαμε το κάστρο χτες. = We saw the castle yesterday.
  This sounds more emphatic, like contrasting we with someone else.

Because βλέπω (I see) has an irregular past form.

The verb works like this:

• βλέπω = I see / I am seeing
• είδα = I saw

So the past stem is not βλεπ- here; it changes to ειδ-.

That means:

• είδα = I saw
• είδες = you saw
• είδε = he/she/it saw
• είδαμε = we saw
• είδατε = you all saw
• είδαν / είδανε = they saw

This is very common in Greek: some verbs have irregular aorist forms.

It is the aorist, which is the normal simple past tense in Greek for a completed action.

In this sentence, Είδαμε το κάστρο χτες, the idea is simply that the action happened yesterday and is complete: We saw the castle yesterday.

A useful contrast is:

• βλέπαμε = we were seeing / we used to see
• είδαμε = we saw

So είδαμε presents the action as a whole, not as ongoing or habitual.

Το is the definite article meaning the, and κάστρο means castle.

So:

• το = the
• κάστρο = castle
• το κάστρο = the castle

Greek nouns have grammatical gender, and κάστρο is neuter, so it takes the neuter article το in the singular.

Yes. In this sentence, το κάστρο is the direct object of είδαμε, so it is in the accusative case.

For this particular noun, the nominative and accusative forms are the same:

• nominative: το κάστρο
• accusative: το κάστρο

That is common with many neuter nouns in Greek. So even though the case is accusative here, the form does not change.

Χτες means yesterday.

It is a very common everyday form. You will also see χθες, which is a more traditional or formal spelling/form.

So:

• χτες = yesterday, common in speech and informal writing
• χθες = yesterday, also correct, often felt to be a bit more formal or standard-looking

Both are widely understood.

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

All of these are possible:

• Είδαμε το κάστρο χτες.
• Χτες είδαμε το κάστρο.
• Το κάστρο το είδαμε χτες.

They all mean roughly We saw the castle yesterday, but the emphasis can shift depending on the order.

For example:

• Χτες είδαμε το κάστρο puts more focus on yesterday
• Το κάστρο το είδαμε χτες puts more focus on the castle

The original sentence is a very natural neutral order.

A rough pronunciation is:

EE-tha-me to KAH-stro htes

A bit more carefully:

• Είδαμε = EE-tha-me
  The δ in Modern Greek sounds like th in this
• το = to
• κάστρο = KAH-stro
• χτες = a sound like htes, with χ like the ch in German Bach or Scottish loch

A more phonetic approximation is: EE-tha-me to KAH-stro chtes

Greek uses accent marks to show which syllable is stressed.

• Είδαμε → stress on Εί
• κάστρο → stress on κά
• χτες is one syllable, so no accent is needed there

Stress matters in Greek pronunciation, so the accents are important when reading and speaking.

Normally, no. Είδαμε is simply we saw.

English often uses the present perfect (we have seen) where Greek uses other structures or just the past depending on context. But in a basic sentence like this, είδαμε is best understood as we saw.

If you want a true perfect-like meaning in Modern Greek, Greek usually expresses that differently, often with έχω + infinitive-like form:

• Έχουμε δει το κάστρο. = We have seen the castle.

So:

• Είδαμε το κάστρο χτες. = We saw the castle yesterday.
• Έχουμε δει το κάστρο. = We have seen the castle.

Yes. That would mean We saw a castle yesterday.

The difference is:

• το κάστρο = the castle
• ένα κάστρο = a castle / one castle

So the original sentence refers to a specific castle, while ένα κάστρο would sound indefinite.

Yes, it is completely natural.

Είδαμε το κάστρο χτες. sounds like normal everyday Greek and would be understood immediately.

A speaker might also say:

• Χτες είδαμε το κάστρο.

Both are very normal. The choice depends mostly on what the speaker wants to emphasize first.