Ο ξεναγός μάς είπε ότι το μουσείο είναι δωρεάν την Κυριακή.

No. Ξεναγός is a common-gender noun: the noun itself stays the same for a male or female guide, and the article shows the gender.

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

So in your sentence, ο tells you the guide is male.

It means us.

With λέω / είπα, Greek can express both say and tell:

• είπε ότι... = he/she said that...
• μας είπε ότι... = he/she told us that...

So μάς is the person receiving the information: to us / us.

In grammar terms, this is the indirect-object pronoun. With μας, the form looks the same as the accusative form, but with this verb Greek normally uses the same pattern as:

• μου είπε = told me
• σου είπε = told you
• τους είπε = told them

That is a very common question.

You will often see μας without an accent. Some writers use μάς to make the sentence easier to read or to add emphasis/contrast.

Here, the accent helps prevent a momentary confusion with μας as a possessive:

• ο ξεναγός μας = our guide
• ο ξεναγός μάς είπε = the guide told us

So the accent does not create a different basic meaning; it mainly helps the reader.

Είπε is the 3rd person singular aorist of λέω.

• λέω = I say / I tell
• είπα = I said / I told
• είπε = he/she said / told

This is an irregular past form, so it is worth memorizing.

Here the aorist shows a single completed action in the past: the guide told us.

Yes. Here ότι means that and introduces the content of what was said.

• μας είπε ότι...
• μας είπε πως...

Both are common, and in many contexts they are interchangeable.

Also, do not confuse ότι with ό,τι:

• ότι = that
• ό,τι = whatever / anything that

So in this sentence, ότι is just the conjunction introducing reported speech.

Because ότι introduces a normal statement, not a subjunctive idea.

After ότι, Greek uses an ordinary clause:

• είπε ότι το μουσείο είναι δωρεάν

Using να είναι would change the meaning and sound unnatural here, because να usually introduces things like desire, possibility, purpose, or something not simply stated as a fact.

So after είπε ότι..., είναι is exactly what you expect.

Greek does not backshift tenses as automatically as English sometimes does.

If the reported statement is still true, or is presented as a general fact, Greek often keeps the present tense:

• Μας είπε ότι το μουσείο είναι δωρεάν την Κυριακή.

That sounds natural because the speaker is reporting information that is still valid.

If you wanted to emphasize that it was true only at that past time, you could use ήταν instead. But in sentences like this, είναι is very normal.

Δωρεάν means free of charge / gratis.

It is an indeclinable word, meaning its form does not change for gender, number, or case. So you can use the same form with many different nouns:

• το μουσείο είναι δωρεάν
• η είσοδος είναι δωρεάν
• τα μαθήματα είναι δωρεάν

Learners can think of it as working like a predicate adjective after είναι, even though its behavior is special because it does not change form.

Also, it means free in the sense of costing nothing, not free in the sense of available or not busy.

Greek often uses the accusative for expressions of time, and days of the week usually appear with the article.

So:

• την Κυριακή = on Sunday

Literally it looks like the Sunday, but idiomatically it means on Sunday.

The form την is used because Κυριακή is a feminine noun.

It can mean either, depending on context.

• It can mean on Sunday = one relevant Sunday
• It can also mean on Sundays = a regular schedule or recurring fact

In a sentence about museum admission, many people might understand it as a general rule unless the surrounding context points to one specific Sunday.

Greek word order is fairly flexible, but not completely free.

The sentence as written,

Ο ξεναγός μάς είπε ότι το μουσείο είναι δωρεάν την Κυριακή

is a neutral, natural order.

You could also hear something like:

• Μας είπε ο ξεναγός ότι το μουσείο είναι δωρεάν την Κυριακή

That might sound a bit different in emphasis, depending on context.

One important point: weak pronouns like μας usually stay very close to the verb, and normally come before a finite verb such as είπε. So while the subject and other parts can move around, μας είπε is the expected pairing.