Кактусу нужно меньше воды, чем орхидее, потому что он любит сухую почву.

Because Russian uses the dative case with нужно in this pattern.

The structure is:

• кому? — the one who needs something → dative
• что? — the thing needed

So:

• Кактусу нужно меньше воды = The cactus needs less water

Literally, it is closer to To the cactus, less water is necessary.

That is why you get:

• кактус → nominative
• кактусу → dative

Нужно means needed / necessary and is very often used to express need in Russian.

So:

• Мне нужно время = I need time
• Кактусу нужно меньше воды = The cactus needs less water

This is a very common Russian way to say need, and it does not work like the English verb to need.

A useful pattern is:

• кому-то нужно что-то = someone needs something

Because меньше normally takes the noun in the genitive.

Compare:

• вода = nominative
• воды = genitive singular

So:

• меньше воды = less water
• больше воды = more water
• много воды = a lot of water

This is very common in Russian after words of quantity and comparison.

Because it matches the same grammar as кактусу.

The full idea is:

• Кактусу нужно меньше воды, чем орхидее нужно
• literally: The cactus needs less water than the orchid needs

Russian often leaves out repeated words, so the second нужно is omitted.

Since орхидея is also the thing that needs something, it also goes into the dative:

• орхидея → nominative
• орхидее → dative

So чем орхидее means than the orchid / more literally than to the orchid in this construction.

Here чем means than.

It introduces a comparison:

• меньше воды, чем орхидее = less water than the orchid

Other examples:

• больше, чем... = more than...
• меньше, чем... = less than...

So in this sentence, чем connects the two sides of the comparison:

• cactus → less water
• orchid → more water than the cactus, or equivalently the cactus needs less than the orchid

The comma before потому что is straightforward: потому что means because, and it introduces a subordinate clause.

• ..., потому что он любит сухую почву.

The comma before чем appears because it introduces the comparative part:

• меньше воды, чем орхидее

In normal writing, this comma is expected here.

So the punctuation helps separate:

1. the main statement
2. the comparison
3. the reason

Because кактус is a masculine noun, and Russian pronouns agree with the noun’s grammatical gender.

• кактус → masculine
• so the pronoun is он = he / it

In English, we normally say it for a plant. In Russian, the same pronoun form used for he is also used for masculine inanimate nouns.

So:

• кактус ... он
• стол ... он
• дом ... он

This is about grammar, not about saying the cactus is literally male.

Because it is the direct object of любит.

The verb любить takes the accusative case:

• любить что? = to love/like what?

Base forms:

• сухая почва = dry soil (nominative)

In the accusative:

• сухую почву

So:

• он любит сухую почву = it likes dry soil

Both words change because they are feminine singular direct objects:

• сухая → сухую
• почва → почву

Not really. Here меньше functions as a comparative/quantifying word, meaning less.

It does not agree like a normal adjective would, and it is commonly followed by the genitive:

• меньше воды
• меньше времени
• меньше денег

So it is best to learn меньше as part of the pattern:

• меньше + genitive noun

Yes, любить literally means to love, but in Russian it is also commonly used in a broader sense like to like or to prefer, especially in general statements.

So here:

• он любит сухую почву

can naturally mean something like:

• it likes dry soil
• it prefers dry soil

In a sentence about a cactus, this sounds normal.

Yes. Russian word order is much more flexible than English word order.

This sentence starts with кактусу because that is the topic being discussed: the cactus.

You could also say things like:

• Орхидее нужно больше воды, чем кактусу.
• Меньше воды нужно кактусу, чем орхидее.

These versions are grammatically possible, but they may sound different in emphasis.

The original sentence is natural because it begins with the thing being described:

• Кактусу нужно меньше воды...

Because water is treated as a mass noun, just like in English.

So Russian normally uses singular:

• вода = water
• воды = of water / some water / water in the genitive

After меньше, you do not use a plural form here. You say:

• меньше воды = less water

not a plural equivalent of waters.

Yes, but нужно is one of the most natural and common choices.

Other possibilities include:

• Кактусу требуется меньше воды = The cactus requires less water
• Кактус нуждается в меньшем количестве воды = The cactus needs a smaller amount of water

These are more formal or heavier in style.

For everyday clear Russian, Кактусу нужно меньше воды is simple and very natural.