ضوء الشمس مهم للنبات المنزلي.

Because this is a nominal sentence in Arabic. In the present tense, Arabic usually leaves out the verb to be.

So:

• ضوء الشمس مهم للنبات المنزلي
• literally: Sunlight important for the houseplant
• natural English: Sunlight is important for the houseplant

If you wanted was, will be, etc., Arabic would use a verb, but for simple present statements it usually does not.

It literally means the light of the sun.

This is an iḍāfa construction, often called a genitive/possessive construction:

• ضوء = light
• الشمس = the sun

Together:

• ضوء الشمس = sunlight / the light of the sun

Arabic often expresses ideas this way instead of making a single compound noun.

Because in an iḍāfa construction, the first noun usually does not take الـ.

So:

• ضوء الشمس is correct
• not الضوء الشمس

Even without الـ, ضوء becomes definite here because it is linked to the definite noun الشمس. So ضوء الشمس means the sunlight or simply sunlight, not just any random light.

Because ش is a sun letter.

In Arabic, when الـ comes before a sun letter, the l sound of الـ is not pronounced. Instead, the next consonant is doubled.

So:

• written: الشمس
• pronounced: ash-shams

The spelling stays the same, but the pronunciation changes.

Because مهم refers back to ضوء, not to الشمس.

The main subject of the sentence is:

• ضوء الشمس = sunlight

The head noun there is ضوء, and ضوء is masculine. So the predicate adjective is also masculine:

• مهم = important

It is not feminine because Arabic agreement follows the main noun being described, and here that noun is ضوء, not الشمس.

Here لـ means for.

So:

• للنبات المنزلي = for the houseplant / for the indoor plant

The basic idea is:

• مهم للنبات المنزلي = important for the houseplant

This لـ is very common in Arabic and can mean for, to, or belonging to, depending on context.

Because when لِـ is followed by a noun with الـ, the forms combine in writing.

So:

• لِ + النبات becomes
• لِلنبات

Also, because ن is a sun letter, the pronunciation is closer to:

• lin-nabāt

So you will see:

• written: للنبات
• pronounced: lin-nabāt

This is a very common spelling pattern with prepositions like لِـ before definite nouns.

It means the houseplant or the indoor plant.

Breakdown:

• النبات = the plant
• المنزلي = domestic, home-related, indoor

So النبات المنزلي is literally something like:

• the home plant
• more natural English: the houseplant / the indoor plant

The adjective المنزلي comes from منزل meaning house/home.

Because adjectives in Arabic agree with the noun they describe in definiteness, as well as gender, number, and case.

Since the noun is definite:

• النبات = the plant

the adjective must also be definite:

• المنزلي = the indoor/domestic

So:

• النبات المنزلي = the indoor plant

If the noun were indefinite, the adjective would also be indefinite.

A fully vocalized version would be:

ضَوْءُ الشَّمْسِ مُهِمٌّ لِلنَّبَاتِ المَنْزِلِيِّ.

Those endings show the grammar more clearly:

• ضَوْءُ = nominative, because it is the subject
• الشَّمْسِ = genitive, because it is the second part of the iḍāfa
• مُهِمٌّ = nominative, because it is the predicate
• لِلنَّبَاتِ = genitive after the preposition لِ
• المَنْزِلِيِّ = genitive because it describes النبات

In normal everyday Arabic writing, these short vowels are usually omitted.

A careful pronunciation is:

ḍawʾu ash-shamsi muhimmun lin-nabāti al-manziliyyi

A simpler learner-friendly pronunciation without full case endings is:

ḍaw’ ash-shams muhimm lin-nabāt al-manzilī

A few pronunciation notes:

• ض is a heavy/emphatic d
• ء in ضوء is a glottal stop
• الشمس is pronounced ash-shams
• للنبات is pronounced lin-nabāt

It can depend on context.

Grammatically, النبات المنزلي is definite singular, so it can mean:

• the houseplant / the indoor plant

But Arabic also sometimes uses a definite singular noun in a more generic sense. So depending on context, the sentence could be understood more broadly as:

• Sunlight is important for houseplants

If there is no context, the safest direct translation is Sunlight is important for the houseplant or for the indoor plant.