Το ότι μας συμφέρει να φύγουμε νωρίς είναι ξεκάθαρο, αφού το απόγευμα έχει πάντα κίνηση.

Το ότι means the fact that.

So:

• Ότι μας συμφέρει να φύγουμε νωρίς = that it is in our interest to leave early
• Το ότι μας συμφέρει να φύγουμε νωρίς = the fact that it is in our interest to leave early

In this sentence, the whole clause is being treated like a noun phrase and used as the subject of είναι ξεκάθαρο:

• Το ότι ... είναι ξεκάθαρο = The fact that ... is clear

Without το, ότι usually just introduces a content clause after another verb, like Ξέρω ότι... = I know that...

So here το is important because it turns the clause into a more noun-like expression.

No. They are different words.

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

In your sentence, it is ότι without the comma-like extra mark, because it introduces a clause:

• Το ότι μας συμφέρει... = The fact that it suits/us benefits us...

Compare:

• Ξέρω ότι έρχεται. = I know that he is coming.
• Πάρε ό,τι θέλεις. = Take whatever you want.

This is a very common distinction for learners.

Συμφέρει is from συμφέρω, and in this kind of sentence it means:

• it is advantageous
• it is in someone’s interest
• it makes sense financially/practically
• sometimes simply it suits

So:

• μας συμφέρει = it is in our interest / it suits us / it benefits us

In this sentence:

• μας συμφέρει να φύγουμε νωρίς = it is in our interest to leave early

This verb is often used impersonally, especially in the 3rd person singular:

• Δεν με συμφέρει. = It’s not worth it for me / It doesn’t suit me.
• Σε συμφέρει να περιμένεις. = It’s in your interest to wait.

Because μας is the object pronoun used with the verb συμφέρει.

Greek says literally something like:

• it benefits us
• it is advantageous to us

So the person affected by συμφέρει is expressed with a weak object pronoun:

• με συμφέρει = it suits me
• σε συμφέρει = it suits you
• μας συμφέρει = it suits us

Using εμείς would give emphasis, but it would not replace μας in normal grammar here.

You could say:

• Εμάς μας συμφέρει να φύγουμε νωρίς.

That means something like:

• As for us, it suits us to leave early.

But in the neutral sentence, μας is the correct form.

Because Greek commonly uses a να-clause where English often uses an infinitive.

English:

• It suits us to leave early

Greek:

• Μας συμφέρει να φύγουμε νωρίς

Modern Greek does not use an infinitive the way English does. Instead, it uses:

• να + verb form

So να φύγουμε corresponds here to to leave.

This is one of the most important structural differences between Greek and English.

Because να φύγουμε uses the aorist subjunctive form, not the present one.

Very roughly:

• να φύγουμε = to leave / to go off as a single complete event
• να φεύγουμε = to be leaving / to leave repeatedly / to keep leaving

In this sentence, we are talking about one specific action:

• leaving early once, on this occasion

So να φύγουμε νωρίς is the natural choice.

If you said να φεύγουμε νωρίς, it would sound more like a repeated or habitual idea:

• It is in our interest to leave early (as a regular practice).

Νωρίς is an adverb, meaning early.

It modifies the verb phrase να φύγουμε:

• να φύγουμε νωρίς = to leave early

Like English early, it does not change form here.

Some useful comparisons:

• νωρίς = early
• αργά = late

Examples:

• Ήρθα νωρίς. = I came early.
• Φύγαμε αργά. = We left late.

Because the subject of είναι is the whole clause Το ότι μας συμφέρει να φύγουμε νωρίς, and Greek treats that kind of clause as a neuter singular idea.

So Greek uses the neuter form:

• ξεκάθαρο = clear

Compare:

• Η απάντηση είναι ξεκάθαρη.
  (The answer is clear.)
  Here απάντηση is feminine, so ξεκάθαρη is feminine.

• Το μήνυμα είναι ξεκάθαρο.
  (The message is clear.)
  Here μήνυμα is neuter, so ξεκάθαρο is neuter.

• Το ότι... είναι ξεκάθαρο.
  A whole clause used as a subject also takes neuter singular agreement.

Here αφού means since / because / given that.

So:

• ..., αφού το απόγευμα έχει πάντα κίνηση.
• ..., since there is always traffic in the afternoon.

It introduces the reason or justification for the statement before it.

Be careful: αφού can have other uses too. In some contexts it can mean something like after, but not in this sentence. Here it clearly means since/because.

Because αφού το απόγευμα έχει πάντα κίνηση is a subordinate clause giving the reason for the main statement.

Greek punctuation often separates this kind of explanatory/reason clause with a comma:

• Το ότι ... είναι ξεκάθαρο, αφού ...

This is similar to English punctuation in a sentence like:

• It’s clear, since there is always traffic in the afternoon.

The comma helps show the structure of the sentence.

Because Greek often uses the definite article with times of day and many time expressions.

So:

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

In this sentence, το απόγευμα means in the afternoon in a general sense.

This is very natural Greek. English usually does not use the in the same way here.

Literally, κίνηση means movement.

But in everyday Greek, έχει κίνηση is a very common expression meaning:

• there is traffic
• it’s busy on the roads

So:

• το απόγευμα έχει πάντα κίνηση = there is always traffic in the afternoon

This is idiomatic Greek. You should learn έχει κίνηση as a set phrase.

Examples:

• Στον δρόμο έχει πολλή κίνηση. = There is a lot of traffic on the road.
• Τέτοια ώρα έχει κίνηση. = There’s traffic at this time.

Both can be possible in Greek, but έχει κίνηση is the more natural everyday expression for traffic.

Greek often uses έχει in impersonal expressions where English uses there is:

• Έχει κόσμο. = There are people / It’s crowded
• Έχει φασαρία. = There is noise
• Έχει κίνηση. = There is traffic

So although υπάρχει κίνηση is grammatically possible, έχει κίνηση sounds more idiomatic in ordinary speech.

The sentence has two main parts:

1. Το ότι μας συμφέρει να φύγουμε νωρίς είναι ξεκάθαρο = The fact that it is in our interest to leave early is clear

2. αφού το απόγευμα έχει πάντα κίνηση = since there is always traffic in the afternoon

So the pattern is:

• [subject clause] + είναι ξεκάθαρο
• then
• [reason introduced by αφού]

A useful way to see it is:

• Subject: Το ότι μας συμφέρει να φύγουμε νωρίς
• Verb: είναι
• Predicate adjective: ξεκάθαρο
• Reason clause: αφού το απόγευμα έχει πάντα κίνηση

Seeing the structure this way can make long Greek sentences much easier to understand.