Kokous alkaa kuudelta, eli meidän pitää lähteä nyt.

Because Finnish uses a special form for clock times when you mean at a certain time.

• kuusi = six
• kuudelta = at six

So:

• Kokous alkaa kuudelta. = The meeting starts at six.

This -lta/-ltä ending is very common with times:

• yhdeltä = at one
• kahdelta = at two
• kolmelta = at three
• kuudelta = at six

So a learner should think of kuudelta as the normal way to say at six o’clock.

Here eli means something like so, that is, or in other words.

In this sentence, it connects the first idea to a conclusion:

• Kokous alkaa kuudelta = the meeting starts at six
• eli meidän pitää lähteä nyt = so we have to leave now

So eli is helping the speaker explain the consequence of the first statement.

It often has a slightly conversational feel. In some situations, Finnish might also use words like joten or siis, but eli is very natural here.

Because with pitää meaning must / have to, Finnish uses the person in the genitive form, not the normal subject form.

So:

• me = we
• meidän = our / of us

But in this structure, meidän does not literally mean our. It marks who has the obligation.

Examples:

• Minun pitää lähteä. = I have to leave.
• Sinun pitää lähteä. = You have to leave.
• Meidän pitää lähteä. = We have to leave.

This is a very common Finnish pattern and is worth memorizing as a whole structure.

Because in this structure, pitää stays in the 3rd person singular form.

So even though the meaning is we have to, Finnish says:

• minun pitää
• sinun pitää
• meidän pitää

The verb does not become something like pidämme here.

That is because this is a fixed grammar pattern:

genitive + pitää + infinitive

So:

• meidän pitää lähteä = we have to leave

This is one of those places where Finnish works quite differently from English.

No. pitää has more than one common meaning.

Two important ones are:

1. to have to / must

   • Meidän pitää lähteä. = We have to leave.

2. to like

   • Pidän kahvista. = I like coffee.

So learners often get confused because the same dictionary form appears in different structures.

In your sentence, pitää clearly means must / have to because it is followed by an infinitive:

• pitää lähteä = have to leave

Because it follows pitää.

After pitää in the sense of must / have to, the next verb is normally in the basic infinitive form:

• pitää mennä = have to go
• pitää tulla = have to come
• pitää lähteä = have to leave

So in:

• meidän pitää lähteä nyt

the main idea is have to leave now, and lähteä stays in the infinitive.

Yes, in meaning it is we, but Finnish does not need to repeat it.

The phrase meidän pitää already tells you who the obligation applies to. So:

• meidän pitää lähteä = we have to leave

Finnish often avoids repeating words that are already clear from the structure.

So there is no separate word for we before lähteä, but the meaning is still clearly we have to leave.

Because the dictionary form of many Finnish verbs is also the same as the 3rd person singular present form.

Here:

• alkaa = dictionary form to begin
• alkaa = begins

Since kokous is singular, the verb is 3rd person singular:

• Kokous alkaa. = The meeting begins.

So even though it looks like the infinitive to an English speaker, here it is a normal finite verb form.

Yes, Finnish word order is fairly flexible, although some orders sound more natural in certain contexts.

The given sentence:

• Kokous alkaa kuudelta, eli meidän pitää lähteä nyt.

is very natural.

But you could also move some parts for emphasis, for example:

• Meidän pitää nyt lähteä, koska kokous alkaa kuudelta.
• Nyt meidän pitää lähteä, koska kokous alkaa kuudelta.

The original version first states the fact, then the conclusion:

1. The meeting starts at six
2. so we have to leave now

That makes it sound logical and smooth.

nyt usually means now.

In this sentence, it means something like:

• now
• right now
• at this point

So:

• meidän pitää lähteä nyt = we have to leave now

Depending on context, it may sound a little urgent, as if it is already time to go.

Because Finnish has no articles like a/an/the.

So:

• kokous can mean a meeting or the meeting

The exact meaning depends on context. In your sentence, English naturally uses the meeting because both speaker and listener probably know which meeting is being discussed.

This is very normal in Finnish, and learners have to get used to understanding definiteness from context rather than from a separate word.