Η ξεναγός μάς έδειξε το πιο παλιό μνημείο της πόλης.

Many -ος nouns are masculine, but not all of them. Ξεναγός can refer to either a male or a female guide, and the article shows the gender:

• ο ξεναγός = the male guide
• η ξεναγός = the female guide

So in this sentence, η tells you the guide is female.

Μάς is the weak object pronoun meaning us.

In Greek, these short object pronouns usually come before a finite verb:

• μάς έδειξε = showed us

This is different from English, where us normally comes after the verb.

A useful pattern:

• με = me
• σε = you
• τον / την / το = him / her / it
• μας = us
• σας = you (plural/formal)
• τους = them

So η ξεναγός μάς έδειξε... literally follows the Greek pattern the guide us showed...

Good question. You will often see μας written without an accent in modern Greek.

Writing μάς can help make it clear that it means us as an object pronoun, not our as a possessive:

• η μητέρα μας = our mother
• η μητέρα μάς είδε = mother saw us

So in your sentence, the accent helps signal us. In many contexts, writers leave it unaccented, but the accented form is very common when clarity is useful.

Έδειξε is the aorist tense, which is the basic Greek past tense for a completed action.

So μάς έδειξε means:

• she showed us
• one completed act in the past

If Greek wanted to emphasize an ongoing or repeated past action, it would use a different form, such as the imperfect:

• μας έδειχνε = she was showing us / she used to show us

The dictionary form is δείχνω = I show.

Its aorist stem changes:

• δείχνω → έδειξα = I showed
• έδειξε = he/she/it showed

So έδειξε is the 3rd person singular aorist.

This kind of stem change is common in Greek verbs, so learners usually memorize both:

• present: δείχνω
• aorist: έδειξα

Yes. Το πιο παλιό μνημείο means the oldest monument.

Here is the pattern:

• παλιό = old
• πιο παλιό = older / more old
• το πιο παλιό = the oldest

So Greek often forms the superlative with: article + πιο + adjective

In more formal Greek, you may also see a one-word form such as το παλαιότερο, but το πιο παλιό is very natural and common.

Because they must agree with each other.

Μνημείο is:

• neuter
• singular

So the article and adjective match it:

• το = neuter singular
• παλιό = neuter singular
• μνημείο = neuter singular noun

Greek adjectives agree with the noun in gender, number, and case.

Because in neuter singular, the nominative and accusative are often identical.

So:

• το μνημείο can be nominative: the monument
• το μνημείο can also be accusative: the monument

In this sentence, it is the direct object because it is the thing the guide showed:

• μάς έδειξε το πιο παλιό μνημείο = she showed us the oldest monument

So the function comes from the sentence structure, not from a visible ending change here.

Της πόλης is the genitive form, meaning of the city.

• η πόλη = the city
• της πόλης = of the city

So:

• το πιο παλιό μνημείο της πόλης = the city's oldest monument
• or more naturally in English: the oldest monument in the city

The genitive is often used in Greek where English uses of or an apostrophe 's.

Greek word order is more flexible than English word order.

The sentence as written is very natural:

• Η ξεναγός μάς έδειξε το πιο παλιό μνημείο της πόλης.

But other orders are also possible, depending on emphasis:

• Μάς έδειξε η ξεναγός το πιο παλιό μνημείο της πόλης.
• Το πιο παλιό μνημείο της πόλης μάς έδειξε η ξεναγός.

These alternatives shift the focus, but the basic meaning stays the same.

For a learner, the original order is a good neutral model: subject + object pronoun + verb + direct object + genitive phrase