Je la kvara mi foriras el la oficejo, kaj je la sepa mi jam estas hejme.

Je is commonly used for clock times in Esperanto.

• je la kvara = at four o’clock
• je la sepa = at seven o’clock

Esperanto often uses je when no more specific preposition is needed. Time expressions with clock hours are a very common case.

You can think of je here as the standard way to say at with exact times.

Esperanto usually tells time with ordinal numbers because the idea is really the fourth hour and the seventh hour.

So:

• la kvara (horo) = the fourth hour = four o’clock
• la sepa (horo) = the seventh hour = seven o’clock

The word horo is often omitted because it is understood.

This is different from English, where we usually say four and seven, not the fourth hour.

Yes. It is short for je la kvara horo.

In normal Esperanto, leaving out horo is very common when the meaning is obvious.

So these mean the same thing:

• je la kvara
• je la kvara horo

The shorter version is natural and very common.

Iri means to go, but foriri means to go away, to leave, or to depart.

So:

• mi iras el la oficejo = I go out of the office
• mi foriras el la oficejo = I leave the office / I go away from the office

The prefix for- adds the idea of movement away from a place.

In this sentence, foriras is a good choice because the speaker is not just moving; they are leaving work for the day.

El means out of or from inside.
De usually means from, but more in the sense of from a person, source, or starting point, not necessarily from inside something.

So:

• el la oficejo = out of the office
• de la oficejo would sound less exact here

Because an office is a place you are physically inside, el is the natural choice.

Jam means already.

So je la sepa mi jam estas hejme means that by seven o’clock, the speaker is already home.

It adds the idea that this has happened by that time, perhaps sooner than expected or sooner than some other point of comparison.

Without jam, the sentence would still make sense, but jam gives it a slightly stronger sense of completion:

• je la sepa mi estas hejme = at seven I am home
• je la sepa mi jam estas hejme = at seven I am already home

Because hejme is an adverb meaning at home.

• hejmo = home as a noun
• hejme = at home as an adverb of place

Esperanto often uses adverbs in -e for locations where English uses phrases like at home, indoors, outside, and so on.

So:

• mi estas hejme = I am at home

You would not normally say mi estas hejmo, because that would try to make home the thing you are, rather than the place where you are.

Grammatically, en la hejmo is possible, but it is less natural here.

• hejme = the normal way to say at home
• en la hejmo = more literally in the home/household

For ordinary everyday meaning, Esperanto strongly prefers hejme.

So mi estas hejme is the standard phrase.

Esperanto often repeats the subject when it starts a new clause, just as English often does.

The sentence has two coordinated clauses:

• je la kvara mi foriras el la oficejo
• kaj je la sepa mi jam estas hejme

Repeating mi makes the structure clear and natural. In some contexts Esperanto can omit repeated material, but here keeping mi is completely normal and stylistically clear.

Both are in the present tense:

• foriras = present tense of foriri
• estas = present tense of esti

Even though the sentence talks about habitual or scheduled times, Esperanto often uses the present tense just as English does:

• At four I leave the office
• At seven I am already home

So this sentence can describe a routine or a regularly true situation.

By itself, je la sepa normally means at seven o’clock.

It usually refers to that time point, but like English at seven, the exactness depends on context. In ordinary speech, it may be understood a little loosely unless the speaker is emphasizing precision.

If someone wanted to be more exact, they could add minutes, for example:

• je la sepa kaj kvarono = at 7:15
• je la sepa kaj duono = at 7:30

But in your sentence, je la sepa is simply the normal way to say at seven o’clock.