El cactus aguanta bien el calor, pero la otra planta empieza a marchitarse si le da demasiado sol.

It is singular here: el cactus = the cactus.

A few useful points:

• Cactus is a singular noun in Spanish, even though it ends in -s.
• The article el shows clearly that it is singular.
• The plural can also be los cactus.

So:

• el cactus = the cactus
• los cactus = the cactuses / the cactus plants

This is one of those nouns whose ending does not follow the most common pattern.

Aguanta comes from aguantar, which often means:

• to withstand
• to tolerate
• to put up with
• to endure

In this sentence, El cactus aguanta bien el calor, it means something like:

• The cactus withstands heat well
• The cactus handles heat well

So it is talking about the cactus being resistant to heat.

Bien means well, so aguanta bien el calor means it handles the heat well.

Without bien:

• aguanta el calor = it can stand the heat

With bien:

• aguanta bien el calor = it stands the heat well / it handles heat well

So bien adds the idea that the cactus does this successfully or easily.

Spanish often uses the definite article more than English does.

So:

• el calor = the heat

Even when English might simply say heat, Spanish often prefers el calor in general statements.

This is very common with nouns used in a broad, general sense:

• No me gusta el frío = I don’t like cold weather / the cold
• El sol es fuerte = The sun is strong
• El cactus aguanta bien el calor = The cactus handles heat well

So the article sounds natural in Spanish.

Both can exist, but la otra planta means the other plant in a specific contrast.

Here the sentence compares two plants:

• El cactus...
• pero la otra planta...

So la otra planta means the other plant, the one that is not the cactus.

If you said otra planta, that would usually mean another plant or a different plant, often less specific.

So:

• la otra planta = the other plant
• otra planta = another plant / a different plant

This is a very common structure:

• empezar a + infinitive = to start/begin to do something

So:

• empieza a marchitarse
• literally: starts to wither

Examples of the same pattern:

• empieza a llover = it starts to rain
• empiezo a entender = I’m starting to understand
• la planta empieza a marchitarse = the plant starts to wilt

This is one of the main ways Spanish says to start doing something.

Because here the verb is being used as an intransitive/reflexive-style verb meaning to wilt or to wither.

• marchitar can mean to wither something or to cause something to wilt
• marchitarse means to wilt / to wither by itself

So:

• El calor marchita la planta = The heat makes the plant wilt
• La planta se marchita = The plant wilts

In your sentence:

• empieza a marchitarse = starts to wilt

The se is part of how this verb is commonly used when the plant itself is undergoing the change.

Literally, it is something like:

• if too much sun gives to it

But in natural English, it means:

• if it gets too much sun
• if too much sun hits it

Here:

• da comes from dar = to give
• le = to it (referring to la otra planta)
• demasiado sol = too much sun

Spanish often uses dar el sol or dar mucho sol to talk about sunlight hitting a place or thing.

For example:

• Aquí da mucho el sol = This spot gets a lot of sun
• Le da el sol a la planta = The sun shines on the plant / the plant gets sun

Le refers to la otra planta.

So in:

• si le da demasiado sol

the idea is:

• if too much sun hits it
• if it gets too much sun

Grammatically, le is an indirect object pronoun meaning to it here.

You could think of the fuller version as something like:

• si demasiado sol le da a la otra planta

But Spanish normally prefers the shorter version with the pronoun.

A useful pattern is:

• A la planta le da el sol = The plant gets sunlight
• A mí me da el sol = The sun is shining on me / I’m getting sun
• A esta ventana le da el sol por la tarde = This window gets sun in the afternoon

Because demasiado here works like a determiner meaning too much.

So:

• demasiado sol = too much sun

You do not normally say demasiado el sol.

Compare:

• mucho sol = a lot of sun
• poco sol = little sun
• demasiado sol = too much sun

These quantity words usually come directly before the noun without an article.

The present tense is used here to express a general truth or habitual situation.

The sentence is not talking about one single moment. It is describing what typically happens:

• the cactus tolerates heat well
• the other plant starts to wilt if it gets too much sun

Spanish uses the present tense for this kind of general statement, just like English often does:

• Cats like warmth
• This plant dies in the cold
• El cactus aguanta bien el calor

So the present tense here means this is generally how these plants behave.

Yes, but the meaning changes slightly.

• se marchita = it wilts
• empieza a marchitarse = it starts to wilt

So empieza a marchitarse focuses on the beginning of the process.

Compare:

• La planta se marchita si le da demasiado sol = The plant wilts if it gets too much sun
• La planta empieza a marchitarse si le da demasiado sol = The plant starts to wilt if it gets too much sun

The original sentence is a bit more precise and suggests the first visible sign of damage.

Yes, very common.

In everyday Spanish, especially when talking about houses, rooms, balconies, gardens, or plants, people often use dar el sol.

Common examples:

• A esta habitación le da mucho el sol = This room gets a lot of sun
• No pongas la planta donde le da el sol directo = Don’t put the plant where it gets direct sunlight
• Por la mañana le da el sol = It gets sun in the morning

So this sentence uses a very natural Spanish way of talking about exposure to sunlight.