Mi promesas, ke mi alvenos frue morgaŭ.

Ke means that and introduces a subordinate clause.

So:

• Mi promesas = I promise
• ke mi alvenos frue morgaŭ = that I will arrive early tomorrow

Esperanto uses ke very regularly to introduce this kind of clause.

In Esperanto, a comma is commonly used before a subordinate clause introduced by words like ke.

So in:

• Mi promesas, ke mi alvenos frue morgaŭ.

the comma helps separate:

• the main clause: Mi promesas
• the subordinate clause: ke mi alvenos frue morgaŭ

This is very standard punctuation in Esperanto.

Because the second clause needs its own subject.

In English, we also say:

• I promise that I will arrive early tomorrow

The second I is necessary, and Esperanto works the same way:

• Mi promesas, ke mi alvenos frue morgaŭ.

You normally cannot leave out the second mi in a full ke-clause.

In Esperanto, verb endings show tense very clearly.

• -as = present
• -is = past
• -os = future

So:

• alvenas = arrive / am arriving
• alvenis = arrived
• alvenos = will arrive

That -os ending is what makes it future.

Because alveni means to arrive, while veni means to come.

They are related, but not identical:

• veni = to come
• alveni = to arrive, to reach a destination

So mi alvenos is the natural way to say I will arrive.

Frue means early.

The -e ending marks an adverb in Esperanto.

Compare:

• frua = early, early-related as an adjective
• frue = early, used adverbially

Here it modifies the verb alvenos, telling us how / when the arrival happens:

• mi alvenos frue = I will arrive early

Morgaŭ means tomorrow.

It is a time word, and here it tells when the action happens:

• mi alvenos morgaŭ = I will arrive tomorrow

In the full sentence:

• frue morgaŭ = early tomorrow

So the speaker is promising that the arrival will happen tomorrow, and specifically early in the day.

No. Esperanto word order is fairly flexible.

You can also say:

• Mi promesas, ke mi morgaŭ alvenos frue.
• Mi promesas, ke mi alvenos morgaŭ frue.

These all mean roughly the same thing, though the emphasis may shift slightly.

The original alvenos frue morgaŭ is natural and clear.

No, that would not be standard Esperanto.

If you use a full clause after promesas, you normally need ke:

• Mi promesas, ke mi alvenos frue morgaŭ.

Without ke, the sentence sounds incomplete or incorrect.

Yes, often you can, especially when the subject is the same in both parts.

So alongside:

• Mi promesas, ke mi alvenos frue morgaŭ.

you can also say:

• Mi promesas alveni frue morgaŭ.

That means essentially the same thing: I promise to arrive early tomorrow.

The ke version is a full clause; the infinitive version is a little more compact.

Yes, that is a very close literal breakdown:

• Mi = I
• promesas = promise
• ke = that
• mi = I
• alvenos = will arrive
• frue = early
• morgaŭ = tomorrow

So the structure is very transparent for an English speaker.

A simple pronunciation guide is:

Mee proh-MEH-sas, keh mee ahl-VEH-nos FROO-eh mor-GOW

A few helpful points:

• c is always like ts
• e is always like eh
• j sounds like English y
• ŭ sounds like a short w-glide

So morgaŭ sounds roughly like MOR-gow.

Also, the stress in Esperanto is normally on the second-to-last syllable:

• proMEsas
• alVEnos
• FRUe
• morGAŬ

Because Esperanto usually expresses tense directly in the verb ending, instead of using extra helper words.

English says:

• I will arrive

Esperanto says:

• mi alvenos

The future meaning is already built into -os, so no separate word like will is needed.