Quand je suis sur le point de sortir, je remarque que la fermeture éclair de ma veste est cassée.

It means to be about to + verb or to be on the verge of + verb.

So je suis sur le point de sortir means I am about to go out / leave.

This is a fixed expression:

• être = to be
• sur le point de = about to
• sortir = go out, leave

Because after sur le point de, French uses the infinitive.

So the pattern is:

• être sur le point de partir
• être sur le point de commencer
• être sur le point de sortir

You do not conjugate the second verb, because the only conjugated verb in that expression is être:

• je suis sur le point de sortir
• tu es sur le point de sortir
• ils sont sur le point de sortir

Because être sur le point de already expresses an action in the immediate future.

So je suis sur le point de sortir is literally present tense, but its meaning is I’m just about to leave.

This is very natural in both French and English:

• When I’m about to leave...
• Quand je suis sur le point de sortir...

Here, quand means when. In this kind of sentence, it can also feel like just as.

So:

• Quand je suis sur le point de sortir...
  can be understood as
• When I’m about to go out...
• Just as I’m about to leave...

The exact nuance depends on context, but when is the basic meaning.

Je remarque que means I notice that.

It is a good choice here because it describes the moment you become aware of something.

Similar options would be:

• je vois que = I see that
• je me rends compte que = I realize that

The differences are small:

• je remarque que = I notice
• je vois que = I see
• je me rends compte que = I realize / become aware

So this sentence has the natural sense of suddenly noticing the problem.

Yes. La fermeture éclair is a common way to say zipper in French.

You may also hear:

• un zip = a zip / zipper
• sometimes just la fermeture if the context is clear

So:

• la fermeture éclair de ma veste = the zipper on my jacket

De ma veste means of my jacket or more naturally in English, on my jacket.

French often uses de where English would use of or sometimes express the relationship differently:

• la porte de la maison = the door of the house / the house’s door
• la fermeture éclair de ma veste = the zipper of my jacket / the zipper on my jacket

It tells you which zipper is broken: the one belonging to my jacket.

Because cassée agrees with la fermeture éclair, which is feminine singular.

The verb phrase is:

• est cassée = is broken

The adjective/past participle cassé changes to match the noun:

• cassé = masculine singular
• cassée = feminine singular
• cassés = masculine plural
• cassées = feminine plural

Since fermeture éclair is feminine singular, French uses cassée.

Because the gender of the whole expression comes from fermeture, the main noun, and fermeture is feminine.

So even though éclair by itself is masculine, the full noun phrase is:

• la fermeture éclair

That is why you get:

• la fermeture éclair est cassée

In this sentence, est cassée means is broken.

It describes the condition of the zipper, not the action of it breaking at that moment.

So:

• la fermeture éclair est cassée = the zipper is broken

If you wanted to say the zipper broke, you would usually say something like:

• la fermeture éclair s’est cassée
• la fermeture éclair a cassé (less common depending on context)

Here, the sentence is clearly about the zipper’s state: it is broken.

Yes. Sortir can mean either, depending on context.

In this sentence:

• Quand je suis sur le point de sortir...

it could mean:

• when I’m about to go out
• when I’m about to leave

Because the next part mentions a jacket, the idea is probably going out / heading outside, but leave is also a perfectly natural English translation.