Mi frapis ĉe la pordo, sed neniu respondis.

Both are in the past tense. In Esperanto, the ending -is always marks a past action.

• frapi → frapis
• respondi → respondis

That is one of the nice regular features of Esperanto: the tense ending stays the same no matter who the subject is.

Yes, frapi basically means to strike, hit, or knock. In the context of a door, it naturally means to knock.

So frapis ĉe la pordo is understood as knocked at the door, not as some violent blow unless the context suggests that.

Here ĉe means something like at or by. So ĉe la pordo means at the door.

Pronunciation:

• ĉ sounds like ch in chair
• e is like e in bet

So ĉe sounds roughly like cheh.

Because this sentence uses a prepositional phrase: ĉe la pordo = at the door.

The speaker is describing where the knocking happened, not making pordo the direct object of the verb in this version.

Esperanto often allows different ways to express similar ideas, and this one is a very natural way to say knocked at the door.

It does not take -n because it follows the preposition ĉe.

In Esperanto, -n is mainly used for:

• a direct object
• or sometimes direction toward something

But after ĉe, you normally just use the basic noun form:

• ĉe la pordo

If the sentence were Mi frapis la pordon, then pordon would have -n because it would be the direct object.

Yes. That would also be understandable, and many learners would naturally produce it.

The difference is mostly one of viewpoint:

• Mi frapis ĉe la pordo = I knocked at the door
• Mi frapis la pordon = I struck/knocked the door

The version with ĉe sounds a bit more like the English idea knock at the door, while la pordon can sound a bit more direct and physical.

Because it is a specific door, not just any door.

Esperanto uses la much like English the. In this sentence, the speaker means a particular door, probably the one they went to.

So:

• la pordo = the door
• pordo = a door / door in a more general sense

Neniu means no one, nobody, or not a single person.

It is used because the sentence is talking about a person answering.

Compare:

• neniu = nobody / no one
• nenio = nothing

So nenio respondis would mean nothing answered, which is not what is meant here.

Because respondi can work by itself, just like English to answer or to respond.

So:

• neniu respondis = nobody answered / nobody responded

If you want to say who was answered, you can add that:

• Neniu respondis al mi = Nobody answered me

In this sentence, that extra information is simply left out because it is already obvious from the context.