La porte est coincée, alors elle ne s’ouvre plus.

Because la porte is a feminine singular noun. In French, words like coincé often agree with the noun they describe.

• masculine singular: coincé
• feminine singular: coincée
• masculine plural: coincés
• feminine plural: coincées

So la porte est coincée means the word describing porte has to be feminine singular too.

Coincée means stuck, jammed, or wedged. For a door, it suggests that something is preventing it from moving properly.

It is stronger and more specific than just closed. A door can be fermée and still work normally, but if it is coincée, something is wrong.

Est coincée is using être + adjective/past participle to describe the door’s condition.

So:

• La porte est coincée = The door is stuck / jammed

This describes a state, not an action.

If you said coince by itself, that would usually be a form of the verb coincer, meaning to jam or to trap, and it would not fit this sentence in the same way.

In French, once the noun has been introduced, it is very common to refer back to it with a pronoun.

• La porte = the door
• elle = it, referring to la porte

French nouns have grammatical gender, so since porte is feminine, the pronoun is elle.

English says it, but French uses il or elle depending on the noun’s gender.

Because the verb here is s’ouvrir, a very common French verb meaning to open in the sense of to open by itself / to be opening / to get opened.

So:

• ouvrir = to open
• s’ouvrir = to open up / to open

In this sentence, elle ne s’ouvre plus means it doesn’t open anymore.

For a door, French often uses s’ouvrir when talking about whether it opens.

The s’ is the shortened form of se, the reflexive pronoun.

• se ouvre becomes s’ouvre before a vowel sound

This apostrophe is just like:

• je aime → j’aime
• le arbre → l’arbre

So s’ouvre is simply se + ouvre contracted.

Ne ... plus means no longer or not anymore.

So:

• elle ne s’ouvre plus = it no longer opens / it doesn’t open anymore

This is a very common negative structure in French.

Compare:

• elle s’ouvre = it opens
• elle ne s’ouvre plus = it no longer opens

In formal written French, yes, you keep ne.

In everyday spoken French, many speakers drop ne, so you may hear:

• Elle s’ouvre plus

But in writing, and especially for learners, elle ne s’ouvre plus is the standard form.

Here alors means so, therefore, or as a result.

It connects the two ideas:

• La porte est coincée = The door is jammed
• alors elle ne s’ouvre plus = so it no longer opens

It shows consequence: the second thing happens because of the first.

Yes, often you could. Both can mean so or therefore.

• La porte est coincée, alors elle ne s’ouvre plus.
• La porte est coincée, donc elle ne s’ouvre plus.

Both are natural.
Very roughly:

• alors can feel a bit more conversational
• donc can feel a bit more direct or logical

But in this sentence, either works well.

French often uses the present tense for things that are true now.

So elle ne s’ouvre plus literally uses the present, but it means that from now on / anymore, the door does not open.

This is normal:

• Il ne vient plus = He doesn’t come anymore
• Je ne fume plus = I don’t smoke anymore

The present tense plus ne ... plus expresses a change from before to now.

In everyday speech, it would often sound like:

• La porte est coincée, alors elle s’ouvre plus.

That is because many speakers drop ne when talking informally.

Also, plus in a negative sentence like this is usually pronounced without the final s sound.

So the formal written version is:

• La porte est coincée, alors elle ne s’ouvre plus.

And the casual spoken version is often:

• La porte est coincée, alors elle s’ouvre plus.