Μερικά λουλούδια ανθίζουν γρήγορα, αλλά άλλα μαραίνονται αν δεν έχουν αρκετό ήλιο.

Because λουλούδια is a neuter plural noun, so the word meaning some has to agree with it.

• singular: ένα λουλούδι = a flower
• plural: μερικά λουλούδια = some flowers

Greek adjectives and determiner-like words usually agree with the noun in:

• gender
• number
• case

So here:

• μερικά = neuter plural
• λουλούδια = neuter plural

That is why μερικοί would be wrong here: μερικοί is masculine plural.

The dictionary form is λουλούδι = flower.

In the sentence, λουλούδια is the plural form.

So:

• το λουλούδι = the flower
• τα λουλούδια = the flowers

This is a very common neuter noun pattern in Greek.

Greek often leaves out a noun when it is already understood from context.

So:

• Μερικά λουλούδια ... αλλά άλλα ...

literally means:

• Some flowers ... but others ...

Here άλλα is neuter plural, matching the understood noun λουλούδια.

So άλλα means other ones / others.

English does the same thing:

• Some flowers bloom quickly, but others wither...

Both are 3rd person plural present forms, but they belong to different verb patterns.

• ανθίζουν = they bloom
• μαραίνονται = they wither

The key difference is that μαραίνονται has a middle/passive-style ending (-ονται / -νται type pattern), while ανθίζουν has an active ending.

This is normal in Greek: not all intransitive verbs use the same endings.

So even though both verbs describe what flowers do, they are built differently.

Not necessarily. It is in a middle/passive form, but here its meaning is simply wither.

So in this sentence:

• άλλα μαραίνονται = others wither

This is an important point for learners: Greek middle/passive verb forms do not always translate as English passives.

Sometimes they are:

• true passives
• reflexive-like meanings
• intransitive verbs with no direct English passive equivalent

Here it is just the natural verb for wither.

Here γρήγορα is an adverb, meaning quickly.

It tells us how the flowers bloom:

• ανθίζουν γρήγορα = bloom quickly

This can confuse learners because γρήγορα also looks like a neuter plural adjective form. But in this sentence, its job is adverbial.

A useful pattern:

• γρήγορος = quick (masculine adjective)
• γρήγορη = quick (feminine adjective)
• γρήγορο = quick (neuter adjective)
• γρήγορα = quickly / fast (adverb)

Because they do different jobs:

• αν = if
• δεν = not

So:

• αν δεν έχουν αρκετό ήλιο = if they do not have enough sun

This is completely normal in Greek.
αν introduces the condition, and δεν negates the verb.

Compare:

• αν έχουν ήλιο = if they have sun
• αν δεν έχουν ήλιο = if they do not have sun

The subject is still άλλα, meaning other flowers.

Greek often does not use an explicit subject pronoun when the verb ending already shows the person and number.

So the structure is:

• άλλα μαραίνονται = other ones wither
• αν δεν έχουν αρκετό ήλιο = if they do not have enough sun

The they is understood from the verb form έχουν and from the earlier noun.

Because έχουν takes a direct object, and direct objects in Greek normally go in the accusative.

The noun is:

• ο ήλιος = the sun

But after have, it becomes accusative:

• έχουν ήλιο = they have sun / sunlight

So:

• αρκετό agrees with ήλιο
• both are masculine singular accusative

That is why you get:

• αρκετό ήλιο = enough sun

not:

• αρκετός ήλιος (which would be nominative, the form used for a subject)

Because ήλιος is the nominative form, while ήλιο is the accusative form.

Greek masculine nouns often change like this:

• ο ήλιος = the sun
• τον ήλιο = the sun (object form)

In this sentence there is no article, but the noun is still in the accusative:

• αρκετό ήλιο

So the missing -ς is a normal case change, not a shortened word.

Because αλλά means but and introduces a contrasting clause.

The sentence has two main parts:

• Μερικά λουλούδια ανθίζουν γρήγορα
• αλλά άλλα μαραίνονται αν δεν έχουν αρκετό ήλιο

The comma helps mark that contrast, just like in English:

• Some flowers bloom quickly, but others wither...

It can mean a general truth or a habitual fact, not only something happening right now.

So this sentence is not necessarily saying:

• these flowers are blooming and withering at this exact moment

It can also mean:

• this is what some flowers generally do

That is very common in both Greek and English. The present tense often expresses:

• habits
• general characteristics
• scientific or natural facts

A simple breakdown is:

• Μερικά λουλούδια = subject
• ανθίζουν = verb
• γρήγορα = adverb

then:

• αλλά = but
• άλλα = subject
• μαραίνονται = verb
• αν δεν έχουν αρκετό ήλιο = conditional clause

So the pattern is:

• Some flowers bloom quickly, but others wither if they do not have enough sun.

This is a very useful model for building your own Greek sentences:

• subject + verb + adverb, αλλά subject + verb + αν clause