La lumière indirecte garde le feuillage sain.

In French, nouns usually need an article. Also, French often uses the definite article for general ideas where English may use no article.

So La lumière indirecte can mean indirect light in a general sense.

Compare:

• La patience est importante. = Patience is important.
• Le sport est bon pour la santé. = Exercise is good for health.

In French, most adjectives come after the noun. Indirecte is one of the adjectives that normally follows the noun, so:

• la lumière indirecte

not usually:

• l’indirecte lumière

A small group of common adjectives often comes before the noun, but indirect is not one of them.

Because lumière is feminine singular, the adjective has to agree with it.

• masculine singular: indirect
• feminine singular: indirecte
• masculine plural: indirects
• feminine plural: indirectes

So:

• un éclairage indirect
• une lumière indirecte

Garde is the present-tense form of the verb garder.

Here it means keeps, not guards.

So:

• La lumière indirecte garde le feuillage sain.
• literally: Indirect light keeps the foliage healthy.

It is the il/elle/on form, matching la lumière:

• je garde
• tu gardes
• il/elle garde
• nous gardons
• vous gardez
• ils/elles gardent

Feuillage means foliage or leafy growth. It is a collective noun, so French often uses it in the singular even though it refers to many leaves.

So:

• le feuillage = the foliage
• les feuilles = the leaves

Both are possible in different contexts, but feuillage sounds more natural when talking generally about a plant’s leafy part.

Because sain agrees with feuillage, not with lumière.

In this sentence:

• la lumière indirecte = the subject
• le feuillage = the thing being kept healthy

Since feuillage is masculine singular, the adjective is masculine singular too:

• le feuillage sain

If the noun were feminine, you would use saine:

• la plante saine

Because French uses the pattern:

garder + object + adjective

This means to keep something + adjective.

So:

• garder le feuillage sain = keep the foliage healthy
• garder la porte ouverte = keep the door open
• garder la chambre propre = keep the room clean

You do not need être here.

Yes. It follows a very normal French pattern:

subject + verb + object + adjective/complement

So:

• La lumière indirecte = subject
• garde = verb
• le feuillage = object
• sain = adjective describing the object

That structure is very common in both French and English.

These are two words that many English speakers find tricky.

• feuillage is pronounced roughly fuh-yazh
• sain is pronounced roughly san with a nasal vowel

A few notes:

• In feuillage, the ll sounds like y.
• The ending -age sounds like azh.
• In sain, the final n is not fully pronounced as an English n. It nasalizes the vowel.

A rough full-sentence pronunciation would be:

La lumière indirecte garde le feuillage sain
≈ la lyu-MYER an-dee-REKT gard luh fuh-YAZH san

That is only an approximation, but it can help as a starting point.