Η συνεργάτιδά μου είπε ένα μικρό αστείο, και ξαφνικά η ατμόσφαιρα στο γραφείο έγινε πιο ήρεμη.

In this sentence, μου belongs to η συνεργάτιδά μου, so it means my.

A very helpful clue is the spelling συνεργάτιδά with the extra accent. That shows that μου is an enclitic attached to the noun phrase η συνεργάτιδα. So the structure is:

η συνεργάτιδά μου = my colleague

If μου were going with the verb είπε and meant to me, the sentence would mean something different, and writers often clarify that with μού είπε. So:

• Η συνεργάτιδά μου είπε... = My colleague said...
• Η συνεργάτιδα μού είπε... = The colleague said to me...

Context also helps, but here the accent pattern strongly points to my colleague.

The basic noun is συνεργάτιδα. It is stressed on the antepenultimate syllable.

When a weak pronoun such as μου is added after a word like this, Greek often adds a second written accent to keep the stress pattern clear:

• συνεργάτιδα
• συνεργάτιδά μου

So this is not a different case or a different word. It is the same noun, just written with an extra accent because of the following enclitic μου.

Είπε is the aorist form of λέω, and it presents the action as a single completed event.

That fits this sentence well: the colleague told one small joke, and then the atmosphere changed.

• είπε = she said / she told
• έλεγε = she was saying / she used to say

So είπε ένα μικρό αστείο means one specific joke was told.
If you used έλεγε, it would sound more ongoing, repeated, or descriptive.

Because αστείο is a neuter singular noun here, and the article-like determiner and adjective have to match it.

So:

• ένα = neuter singular
• μικρό = neuter singular
• αστείο = neuter singular

This whole phrase is the direct object of είπε, so it is in the accusative. For neuter singular nouns like αστείο, the nominative and accusative look the same, which is why the form does not change.

Yes, here it is functioning as a noun.

Greek often uses the neuter form of an adjective as a noun. So αστείο can mean:

• as an adjective: funny
• as a noun: joke or something funny

In this sentence, ένα μικρό αστείο clearly means a small joke, so αστείο is a noun.

Ξαφνικά is an adverb, meaning suddenly. It modifies the whole situation that follows: the atmosphere suddenly became calmer.

Its position is fairly flexible. Greek often allows adverbs like this to move depending on rhythm or emphasis. For example, these are all possible:

• και ξαφνικά η ατμόσφαιρα...
• και η ατμόσφαιρα ξαφνικά έγινε...
• και, ξαφνικά, η ατμόσφαιρα...

The version in your sentence is very natural and narratively smooth.

Because στο is the normal contraction of σε + το.

So:

• σε το γραφείο → στο γραφείο

This is extremely common in Greek:

• σε + τη → στη
• σε + το → στο
• σε + τον → στον

Also, after σε, Greek uses the accusative case. Here γραφείο is neuter singular, and its nominative and accusative forms are the same, so you do not see a visible change.

This uses γίνομαι plus an adjective to express a change of state:

• έγινε = became
• πιο ήρεμη = calmer / more calm

So the idea is not just that the atmosphere was calm, but that it became calmer after the joke.

This is a very common Greek pattern:

• έγινε καλύτερος = became better
• έγινε δύσκολη = became difficult
• έγινε πιο ήρεμη = became calmer

Πιο makes the comparative, like more in English.

So:

• ήρεμη = calm
• πιο ήρεμη = calmer / more calm

In Modern Greek, πιο + adjective is the most common everyday way to form comparisons.

Because the adjective has to agree with η ατμόσφαιρα, which is feminine singular.

So the adjective also appears in the feminine singular form:

• ήρεμος = masculine
• ήρεμη = feminine
• ήρεμο = neuter

Since ατμόσφαιρα is feminine, Greek uses ήρεμη.

Not completely. Greek word order is more flexible than English word order, although some orders sound more neutral than others.

The sentence you have is a very natural neutral narrative order:

• subject: Η συνεργάτιδά μου
• verb: είπε
• object: ένα μικρό αστείο
• then the next clause: και ξαφνικά...

But Greek can move elements for emphasis or style. For example, ξαφνικά could move, and sometimes the subject could come after the verb. The meaning would stay basically the same, but the focus or rhythm would change.

So the given order is not the only possible one, but it is a very standard and natural one.