La forto de la vento fermis la pordon.

La means the. In this sentence, both forto and vento are understood as specific:

• la forto = the force
• la vento = the wind

So la forto de la vento means the force of the wind.

Esperanto often uses la when English would too. You can omit la in more general statements, but here the sentence refers to a particular force and a particular wind.

De usually means of, from, or sometimes shows possession or relation.

Here it links the two nouns:

• la forto de la vento = the force of the wind

So de is functioning like English of.

No. Vento is singular because the sentence is talking about the wind as a mass or natural force, not several separate winds.

• vento = wind
• ventoj = winds

English also often says the wind in this kind of sentence, so Esperanto matches that idea closely.

The -n ending marks the direct object in Esperanto.

In this sentence:

• La forto de la vento = the subject, the thing doing the action
• fermis = closed
• la pordon = the object, the thing being closed

So:

• pordo = door
• pordon = door as the direct object

This is one of the most important features of Esperanto grammar.

Because forto is the subject, not the direct object.

The subject is the thing that performs the action:

• La forto de la vento closed something.

The thing affected by the action is the direct object:

• it closed la pordon

So only pordon gets -n.

Fermis is the past tense of fermi, meaning to close.

Esperanto verb endings are very regular:

• fermi = to close
• fermas = closes / is closing
• fermis = closed
• fermos = will close

So fermis means closed.

In Esperanto, -is always marks the past tense.

So:

• fermis = closed
• vidis = saw
• estis = was / were

This ending does not change for different subjects. Whether the subject is I, you, he, they, or the force of the wind, the past tense still ends in -is.

Yes, to some extent. Because Esperanto marks the direct object with -n, word order is more flexible than in English.

The normal order here is:

• La forto de la vento fermis la pordon.

But you could also say:

• La pordon fermis la forto de la vento.

That still means the same thing, because pordon has -n, so it is clearly the object.

Even so, the original version is the most neutral and natural for a beginner.

Because the sentence refers to the door, a specific door.

• pordon = a door, as direct object
• la pordon = the door, as direct object

The -n shows its grammatical role, and la shows that it is definite.

Not exactly in the ownership sense. De can show a relationship that English often expresses with of.

So la forto de la vento means:

• the force belonging to the wind in a descriptive sense
• the force produced by the wind
• the wind’s force

You could think of it as similar to English the wind’s force, but Esperanto usually expresses that with de.

The dictionary forms are:

• forto = force
• vento = wind
• fermi = to close
• pordo = door

In the sentence, some forms change because of grammar:

• fermi → fermis for past tense
• pordo → pordon for direct object

Because de la vento modifies forto and tells you what kind of force it is.

So the phrase breaks down like this:

• la forto = the force
• de la vento = of the wind

Together they form one complete subject:

• La forto de la vento = the force of the wind

The verb fermis then tells what that subject did.

Yes, slightly.

• La vento fermis la pordon = The wind closed the door
• La forto de la vento fermis la pordon = The force of the wind closed the door

The second version emphasizes that it was specifically the wind’s force that caused the door to close. It sounds a bit more explicit or descriptive.