Η συνεργάτιδά μου είπε ότι είναι διαθέσιμη αύριο το βράδυ, οπότε θα κάνουμε μαζί επανάληψη.

Because μου is an enclitic word in Greek. When an enclitic follows a word stressed on the antepenultimate syllable, Greek often adds a second written accent to the final syllable of the first word.

So:

• base form: συνεργάτιδα
• with enclitic μου: συνεργάτιδά μου

This does not mean the word has completely changed stress pattern in the way a new dictionary form would. It is mainly an orthographic/stress-rule effect caused by the enclitic.

You will see the same thing in other phrases, such as:

• ο άνθρωπός μου
• η δασκάλά μου? No, not here, because the stress pattern is different.

So in this sentence, συνεργάτιδά μου is exactly what you should expect.

Συνεργάτιδα is the feminine form of συνεργάτης / συνεργάτιδα.

Depending on context, it can mean:

• colleague
• coworker
• associate
• female collaborator

In this sentence, the most natural meaning is probably female colleague or coworker.

So:

• συνεργάτης = male colleague/collaborator
• συνεργάτιδα = female colleague/collaborator

In Greek, the weak possessive forms such as:

• μου = my
• σου = your
• του / της = his / her

normally come after the noun.

So Greek usually says:

• η συνεργάτιδά μου = my colleague

not a direct word-for-word equivalent of English my colleague.

If you want to add emphasis, Greek can use δικός:

• η δική μου συνεργάτιδα = my own colleague / my colleague with emphasis

But in normal everyday Greek, noun + μου is the standard pattern.

Because Greek often omits subject pronouns when they are understood from context. This is very common.

In English, you normally need:

• She said that she is available

In Greek, once η συνεργάτιδά μου has already been mentioned, you can simply say:

• είπε ότι είναι διαθέσιμη

The understood subject of είναι is still η συνεργάτιδά μου.

You could include αυτή for emphasis or contrast:

• είπε ότι αυτή είναι διαθέσιμη

But that would sound marked, as if you were stressing she in particular.

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

Here, the person being described is η συνεργάτιδά μου, which is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• διαθέσιμος = masculine
• διαθέσιμη = feminine
• διαθέσιμο = neuter

That is why the sentence has:

• είναι διαθέσιμη = she is available

Here ότι means that and introduces a content clause:

• είπε ότι είναι διαθέσιμη
• she said that she is available

This is very common after verbs of saying, knowing, thinking, etc.

You may also see πως used in the same kind of sentence:

• είπε πως είναι διαθέσιμη

In many cases, ότι and πως are both possible here.

A useful distinction in writing is:

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

So be careful not to confuse those two.

Literally, αύριο το βράδυ is something like:

• tomorrow the evening

But in natural English, it means:

• tomorrow evening
• tomorrow night

Greek often uses the definite article with parts of the day:

• το πρωί = in the morning
• το μεσημέρι = at noon / midday
• το απόγευμα = in the afternoon
• το βράδυ = in the evening / at night

So αύριο το βράδυ is a very normal Greek way to say tomorrow night.

In this sentence, οπότε means:

• so
• therefore
• as a result

So the structure is:

• She said she is available tomorrow night, so we will revise together.

Be aware that οπότε can also have a time-related meaning in other contexts, closer to when or whenever. But here it clearly means so / therefore.

By itself, κάνουμε is the present form:

• κάνουμε = we do / we are doing

When you add θα, it becomes future:

• θα κάνουμε = we will do

So Greek forms the future with:

• θα
  • verb

In this sentence:

• θα κάνουμε μαζί επανάληψη
• we will do revision together

This is one of the most basic and important Greek patterns to learn.

Μαζί means together.

So:

• θα κάνουμε μαζί επανάληψη = we will do revision together

Its position is fairly flexible. You could also hear:

• θα κάνουμε επανάληψη μαζί

Both are natural. The version in your sentence places μαζί before επανάληψη, but the meaning stays the same.

Because κάνω επανάληψη is a very common Greek expression meaning:

• to revise
• to do revision
• to review

In this kind of expression, Greek often leaves out the article when talking about the activity in a general sense.

So:

• κάνουμε επανάληψη = we are revising / doing revision

If you added an article, it could sound more specific, as if you meant a particular review session or a specific review of something already known:

• κάνουμε την επανάληψη

But in your sentence, the article-free version is the most natural.

Yes. Greek often allows small variations in word order or connector choice without changing the basic meaning.

For example, you might also hear:

• Η συνεργάτιδά μου είπε πως είναι διαθέσιμη αύριο το βράδυ, οπότε θα κάνουμε επανάληψη μαζί.

This still means the same thing. The original sentence is perfectly natural; these are just normal Greek variations.

That is something English speakers often need to get used to: Greek word order is often a bit more flexible than English word order.