Ο προϊστάμενος μου είπε ότι αύριο θα έχουμε συνάντηση στο γραφείο.

Προϊστάμενος means supervisor, manager, or boss, depending on context.

In this sentence, it most naturally means the boss / the supervisor.

A few useful notes:

• It is masculine singular.
• Here it is in the nominative, because it is the subject of the sentence.
• It is pronounced roughly pro-i-STA-me-nos.

The ϊ is important: it shows that the vowels are pronounced separately.

Ο is the masculine singular definite article, so it means the.

Greek uses the definite article very often, including before professions, roles, and titles when a specific person is meant. So Ο προϊστάμενος is literally the supervisor / the boss.

Here μου means to me, not my.

So:

• μου είπε = told me

This is a very common source of confusion, because μου can also mean my in other sentences:

• ο φίλος μου = my friend

But in your sentence, μου belongs with the verb είπε, so it means to me.

If you wanted to say my boss told me, Greek would normally be:

• Ο προϊστάμενός μου μου είπε...

That has one μου for my and another μου for to me.

Είπε is the aorist form of λέω, and here it means said / told in a completed past action.

So:

• είπα = I said
• είπε = he/she said
• λέει = he/she says or sometimes he/she is saying

In this sentence, the speaker is talking about one completed event in the past, so είπε is the natural form.

Also, Greek λέω can correspond to both say and tell in English.
Because the sentence includes μου = to me, English usually translates it as told me.

Ότι introduces the content of what was said. It corresponds to English that.

So:

• μου είπε ότι... = he told me that...

In everyday Greek, ότι is very common, but speakers may also use πως in the same kind of sentence:

• Μου είπε ότι...
• Μου είπε πως...

Both are normal here.

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

Αύριο means tomorrow, and putting it before θα έχουμε simply brings the time reference in early:

• ότι αύριο θα έχουμε... = that tomorrow we will have...

You could also hear:

• ότι θα έχουμε αύριο συνάντηση...
• ότι θα έχουμε συνάντηση αύριο...

All of these are possible, though some may sound more natural than others depending on emphasis.

Modern Greek usually forms the future with:

• θα
  • non-past verb form

So here:

• θα = future marker
• έχουμε = we have
• θα έχουμε = we will have

Unlike English, Greek does not normally use a separate future verb form the way some languages do. The little word θα does the future job.

The word we is built into the verb ending.

• έχω = I have
• έχεις = you have
• έχει = he/she/it has
• έχουμε = we have
• έχετε = you plural / formal singular have
• έχουν = they have

So θα έχουμε already means we will have. Greek often leaves out subject pronouns like εμείς because the verb ending already tells you who the subject is.

If you add εμείς, it usually adds emphasis:

• Εμείς θα έχουμε συνάντηση... = We will have the meeting... / We are the ones who will have the meeting...

Greek can say this naturally without an indefinite article here.

So:

• θα έχουμε συνάντηση = we will have a meeting

You can also say:

• θα έχουμε μια συνάντηση

That is also correct, but the version without μια is very natural in this kind of expression.

This is especially common with event-type nouns and set expressions, where English uses a, but Greek often does not need to.

Συνάντηση means meeting.

In this sentence, it clearly refers to a planned meeting, probably work-related.

A few grammar points:

• It is a feminine noun
• Its dictionary form is η συνάντηση
• Here it appears without the article, which is normal in this expression

Στο is a contraction of:

• σε
  • το → στο

The preposition σε can mean in, at, or to, depending on context.

So:

• στο γραφείο can mean in the office, at the office, or to the office

In your sentence, because it goes with συνάντηση, the meaning is at the office or in the office, not movement to the office.

Because γραφείο is the noun office, and after σε / στο Greek normally uses the accusative.

Here:

• το γραφείο = the office
• σε το γραφείο → στο γραφείο

A useful detail:

• γραφείο is neuter
• in the singular, the nominative and accusative forms are the same: γραφείο

So even though it is accusative here, it still looks like γραφείο.

Roughly like:

• pro-i-STA-me-nos

Important points:

• οϊ is pronounced in two parts, because of the diaeresis on ϊ
• the stress falls on στά

So do not pronounce it like one single vowel sound. It is:

• pro-i-... not
• proi-... as one merged sound

It can definitely be rearranged. Greek word order is flexible.

For example, these are all possible:

• Ο προϊστάμενος μου είπε ότι αύριο θα έχουμε συνάντηση στο γραφείο.
• Ο προϊστάμενος μου είπε ότι θα έχουμε συνάντηση αύριο στο γραφείο.
• Αύριο, ο προϊστάμενος μου είπε ότι θα έχουμε συνάντηση στο γραφείο.
  This one changes the emphasis a bit and could sound less neutral depending on context.

The original sentence is a very natural, neutral way to say it.

Yes, sometimes, especially in more informal speech.

You may hear:

• Ο προϊστάμενος μου είπε αύριο θα έχουμε συνάντηση στο γραφείο.

But including ότι is clear, standard, and very common. For learners, it is usually safest to include it.

So the version in your sentence is completely normal and often the best model to follow.