Paul ouvre le portail pendant que Marie arrose la haie avec un arrosoir vert.

That is very normal in French. The present tense often covers both:

• simple present: Paul opens
• present progressive: Paul is opening

So Paul ouvre le portail can naturally mean Paul is opening the gate in context.

If you want to strongly emphasize that the action is in progress right now, French can also use:

• Paul est en train d’ouvrir le portail.

But in ordinary descriptions, the simple present is usually enough.

Pendant que means while and links two actions happening at the same time.

So the structure is:

• Paul ouvre le portail
• pendant que
• Marie arrose la haie avec un arrosoir vert

It introduces a whole clause with its own subject and verb: Marie arrose.

A useful contrast:

• pendant
  • noun/time expression
    
    • pendant deux heures
• pendant que
  • full clause
    
    • pendant que Marie arrose la haie

This is a very common pattern.

• le portail and la haie suggest the gate and the hedge are identifiable in the situation. They are treated as known things in the scene.
• un arrosoir introduces a watering can as one item among possible watering cans, not as something already specifically identified.

So the sentence feels like:

• the gate
• the hedge
• a green watering can

French uses articles very often, and in many cases you just learn the natural choice from context.

Because haie has an aspirated h (h aspiré).

That means:

• the h is still not pronounced
• but grammatically it blocks elision and liaison

So you say:

• la haie
• not l’haie

This is something learners often find tricky, because the h is silent but still affects the grammar.

They match the subjects Paul and Marie, which are both third-person singular.

• Paul ouvre = he opens / is opening
• Marie arrose = she waters / is watering

More specifically:

• ouvrir is conjugated as j’ouvre, tu ouvres, il/elle ouvre...
• arroser is a regular -er verb: j’arrose, tu arroses, il/elle arrose...

So these are the correct singular forms for Paul and Marie.

Because vert agrees with arrosoir, and arrosoir is masculine singular.

• un arrosoir vert
• not un arrosoir verte

French adjectives usually agree in gender and number with the noun they describe.

Examples:

• un arrosoir vert
• une bouteille verte
• des arrosoirs verts

In this sentence, it means the watering can is green.

That is because vert is attached to arrosoir:

• avec un arrosoir vert = with a green watering can

If you wanted to say the hedge is green, you would normally say:

• Marie arrose la haie verte
• or Marie arrose la haie, qui est verte

So the current sentence describes the tool as green, not the hedge.

Portail usually means a gate, often a larger outside entrance, such as:

• a garden gate
• a driveway gate
• a main entrance gate

Porte usually means a door.

So le portail is the natural word if Paul is opening an outside gate rather than an indoor door.

Yes. That is completely natural.

For example:

• Pendant que Marie arrose la haie avec un arrosoir vert, Paul ouvre le portail.

This means the same thing. When the pendant que clause comes first, French often uses a comma before the main clause.

So both orders are fine:

• Paul ouvre le portail pendant que Marie arrose la haie avec un arrosoir vert.
• Pendant que Marie arrose la haie avec un arrosoir vert, Paul ouvre le portail.

In this context, yes: arroser means to water plants.

More generally, arroser can mean to sprinkle, to spray, or to water something with liquid.

So with la haie, the natural English meaning is to water the hedge.

It is a useful everyday verb for gardening contexts:

• arroser les fleurs = water the flowers
• arroser le jardin = water the garden