Mi rimarkis, ke la pordo estas ankoraŭ malfermita.

Because -is marks the past tense in Esperanto.

So:

• mi rimarkas = I notice / I am noticing
• mi rimarkis = I noticed

The speaker is talking about a noticing-event that already happened, so rimarkis is the correct form.

Ke means that and introduces a subordinate clause.

So the structure is:

• Mi rimarkis = I noticed
• ke la pordo estas ankoraŭ malfermita = that the door is still open

In English, that is often optional: I noticed (that) the door...
In Esperanto, ke is normally kept.

Because la pordo is the subject of the clause la pordo estas ankoraŭ malfermita.

In Esperanto, the -n ending marks the direct object, not the subject.

Here:

• Mi is the subject of rimarkis
• the whole clause beginning with ke is what was noticed
• inside that clause, la pordo is the subject of estas

So no -n is needed on pordo.

Esperanto does not automatically change the tense of a subordinate clause just because the main verb is past.

The tense in the ke-clause shows the time of that action or state itself.

So:

• Mi rimarkis, ke la pordo estas ankoraŭ malfermita suggests that the door is still open
• Mi rimarkis, ke la pordo estis malfermita would mean that the door was open at that past time

This is often surprising to English speakers because English often shifts tenses in reported statements. Esperanto is more direct here.

Here ankoraŭ means still.

So estas ankoraŭ malfermita = is still open.

Depending on context, ankoraŭ can also mean yet, but in this sentence still is the natural meaning.

Because it naturally modifies the state malfermita.

In Esperanto, adverbs often stand near the word or idea they modify. So:

• estas ankoraŭ malfermita = is still open

That is the most natural neutral order here. Esperanto word order is fairly flexible, but this placement is clear and idiomatic.

Malfermita is a participial adjective from malfermi = to open.

It means something like opened or in an opened state. With doors, windows, boxes, and similar things, this is very commonly used where English simply says open.

• malfermita pordo = an open door

Malferma also exists, but it tends to mean open more as a general quality or characteristic.
For a physical door that has been left open, malfermita is very natural.

Yes. Even though it comes from a verb, it is functioning as an adjective.

It describes la pordo, just like an ordinary adjective would.

That is why it appears after estas:

• la pordo estas malfermita = the door is open

And like other adjectives, it can agree in number and case if needed:

• la pordoj estas malfermitaj = the doors are open

It already has the normal adjective ending -a, but in this sentence it stays in its basic form because:

• pordo is singular
• it is not accusative

So:

• singular nominative: malfermita
• plural nominative: malfermitaj
• singular accusative: malfermitan
• plural accusative: malfermitajn

Here the noun is la pordo, singular subject, so malfermita is the correct form.

Yes, but the given version is very standard and natural.

For example, Esperanto allows some word-order flexibility, but:

• Mi rimarkis, ke la pordo estas ankoraŭ malfermita is the clearest neutral form.

You might also hear slightly different phrasing in other contexts, but a learner should treat this version as a very good model sentence.

Not exactly.

• vidi = to see
• rimarki = to notice

So rimarki usually implies that you became aware of something, not just that it was visible.

In this sentence, Mi rimarkis... suggests I noticed / realized / became aware that..., which fits the idea better than simple seeing.