L’arrosage garde la plante d’intérieur saine.

Because le becomes l’ before a vowel sound. The noun arrosage starts with a-, so le arrosage contracts to l’arrosage.

This is very common in French:

• le arbre → l’arbre
• la école → l’école

Here, l’arrosage is masculine singular, even though you cannot see le anymore because of the elision.

Arrosage is the noun meaning watering or the act/process of watering.

It comes from the verb arroser, which means to water.

So:

• arroser = to water
• l’arrosage = watering

In this sentence, l’arrosage refers to watering as a general activity, not necessarily one single watering event.

Because garde is the conjugated verb, while garder is the infinitive.

The subject of the sentence is l’arrosage, which is third-person singular, so the verb is conjugated as garde:

• je garde
• tu gardes
• il/elle/on garde

So:

• L’arrosage garde... = Watering keeps...

If you used garder, the sentence would be ungrammatical unless it followed another verb, such as:

• Il faut garder...

Because plante is a feminine noun in French.

So you say:

• la plante = the plant

Not:

• le plante

This also affects other words in the sentence, especially the adjective at the end: saine, which is feminine to agree with plante.

D’intérieur means indoor or more literally of interior/inside.

So la plante d’intérieur means:

• the indoor plant
• the houseplant

The de becomes d’ before a vowel, just like le becomes l’:

• de intérieur → d’intérieur

This structure is very common in French:

• une table de cuisine = a kitchen table
• une plante d’intérieur = an indoor plant

Because French often uses a noun phrase after the noun where English uses a pre-noun modifier.

English says:

• an indoor plant

French naturally says:

• une plante d’intérieur

This is normal French word order. You should usually learn plante d’intérieur as a set expression.

Because saine agrees with la plante, which is feminine singular.

The adjective sain / saine means healthy. Its forms are:

• sain = masculine singular
• saine = feminine singular
• sains = masculine plural
• saines = feminine plural

Since the sentence is talking about la plante, French uses the feminine singular form:

• la plante ... saine

Because after verbs like garder, French can use an adjective to describe the object’s resulting or continuing state.

So in:

• L’arrosage garde la plante d’intérieur saine

the structure is:

• L’arrosage = subject
• garde = verb
• la plante d’intérieur = object
• saine = adjective describing that object

In English, this works similarly:

• Watering keeps the indoor plant healthy

So saine is not describing l’arrosage. It describes la plante.

Yes, it is grammatical and understandable. It means that watering keeps the plant healthy.

However, depending on context, French speakers might also say:

• L’arrosage maintient la plante d’intérieur en bonne santé.
• Arroser la plante l’aide à rester en bonne santé.

So the original sentence is fine, but en bonne santé can sound a bit more natural in some everyday contexts than saine, especially for plants.

Here it most likely means watering in general, as a general truth.

French often uses the definite article for general statements:

• Le sport est bon pour la santé.
• La patience est importante.
• L’arrosage garde la plante d’intérieur saine.

So even though it literally looks like the watering, it often corresponds to English watering in a general sense.

A careful pronunciation is roughly:

la-ro-zhaZH gard la plant dan-tay-ree-uhr sen

A few helpful points:

• L’arrosage: the g sounds like the s in measure
• garde: the final e is silent
• plante: the final e is silent
• d’intérieur: the d’ links directly into the next word
• saine sounds like seine and is pronounced like sen in English approximation

There is no strong extra pronunciation for the final e in saine.