Kiam nia instruistino eniris, ni jam estis legintaj la unuan paĝon.

Because estis legintaj shows that the reading was already completed before another past action.

• eniris = entered / came in
• estis legintaj = had read

So this is Esperanto’s way of expressing something like the English past perfect: we had read the first page by the time the teacher entered.

If you said ni jam legis la unuan paĝon, that could also work in many contexts, but ni jam estis legintaj makes the earlier-than-past relationship especially clear.

legintaj is made from:

• leg- = read
• -int- = active past participle, meaning having done
• -a = adjective ending
• -j = plural

So legintaj literally means having read.

In ni jam estis legintaj, it means we were people who had already read the first page. That is why the whole expression functions like we had read.

Because it describes ni, which is plural.

In Esperanto, adjectives and participles used adjectivally agree with the noun or pronoun they describe:

• singular: mi estis leginta
• plural: ni estis legintaj

Since legintaj is describing ni, it has to be plural.

Because legintaj is not the direct object. It is part of the predicate after estis.

In Esperanto, after esti, the describing word usually stays in the basic adjective form and agrees only in number if needed:

• Mi estas laca.
• Ili estas pretaj.
• Ni estis legintaj.

So legintaj gets -j for plural, but not -n.

Because both the adjective and the noun must agree, and the whole phrase is the direct object of legi.

• unuan = first
• paĝon = page

In Esperanto, adjectives agree with nouns in:

• number
• accusative

So:

• la unua paĝo = the first page
• la unuan paĝon = the first page as an object

Both words take -n because the phrase is what was read.

jam means already.

It shows that by the time the teacher entered, the action was already completed. It emphasizes that the reading happened earlier than the teacher’s entry.

So:

• ni estis legintaj = we had read
• ni jam estis legintaj = we had already read

Kiam means when, and it introduces the time clause.

So:

• Kiam nia instruistino eniris = when our teacher entered

This clause sets the reference point in the past. Then the main clause tells what had already happened by that point.

This is a very common pattern:

• Kiam li alvenis, mi jam estis foririnta. = When he arrived, I had already left.

instruisto means teacher.
The suffix -ino marks a female person, so:

• instruisto = teacher
• instruistino = female teacher

So nia instruistino means our female teacher.

A learner may notice that Esperanto often marks female forms explicitly with -ino.

Yes. Eniri means to enter, to go in, or to come in.

So nia instruistino eniris can be understood as:

• our teacher entered
• our teacher came in

In Esperanto, eniri already contains the idea of going into something, so it often works without needing an extra word like into.

It is correct and clear, but in everyday speech many speakers often prefer simpler verb forms if the context already shows the time relationship.

For example, some people might say:

• Kiam nia instruistino eniris, ni jam legis la unuan paĝon.

But ni jam estis legintaj is more precise if you want to stress that the reading was completed before the teacher entered. So it is a good and fully grammatical form, even if it can sound a bit more formal or deliberate.

That is close to the structure, but not good English.

A literal breakdown helps explain the grammar:

• Kiam = when
• nia instruistino = our female teacher
• eniris = entered
• ni jam estis legintaj = we had already read
• la unuan paĝon = the first page

Although legintaj literally means something like having read, the normal English translation of estis legintaj is had read, not were having-read.

Yes, the order can be changed.

The original sentence starts with the time clause:

• Kiam nia instruistino eniris, ni jam estis legintaj la unuan paĝon.

You could also say:

• Ni jam estis legintaj la unuan paĝon, kiam nia instruistino eniris.

Both are grammatical. The difference is mostly about emphasis and style. Starting with Kiam... foregrounds the time setting first.