У этого фикуса крепкий корень и крепкий стебель, поэтому он растёт быстро.

Russian usually expresses has / have with the pattern у + genitive + есть / implied "is".

So У этого фикуса крепкий корень... literally works like At this ficus, there is a strong root..., which is the normal Russian way to say This ficus has a strong root...

Using иметь is grammatically possible, but it is less common in everyday Russian for simple possession and often sounds more formal or bookish.

Because the preposition у requires the genitive case.

So:

• этот фикус = nominative
• этого фикуса = genitive

Both the demonstrative and the noun change:

• этот → этого
• фикус → фикуса

This is a very common pattern:

• У этого дома
• У моего друга
• У нашей кошки

In the present tense, Russian often omits есть in this kind of possession sentence.

So:

• У этого фикуса есть крепкий корень.
• У этого фикуса крепкий корень.

Both are possible, but the second is very natural and common.

In other words, the sentence still means has, even though no separate word for has appears.

They are in the nominative singular.

That is because they are the things that exist / are present in the possession structure:

• У этого фикуса = of/at this ficus
• крепкий корень и крепкий стебель = a strong root and a strong stem

The adjectives agree with the nouns:

• крепкий корень — masculine singular nominative
• крепкий стебель — masculine singular nominative

Yes, Russian can say крепкий корень и стебель if the adjective clearly applies to both nouns.

But repeating the adjective is often natural and clearer:

• крепкий корень и крепкий стебель

This makes it explicit that both the root and the stem are strong/sturdy. Russian often repeats adjectives in coordinated phrases more readily than English does.

So both are possible, but the repeated version sounds clear and fully natural.

A noun ending in -ь can be either masculine or feminine in Russian. The ending alone does not tell you the gender.

For example:

• день — masculine
• словарь — masculine
• дверь — feminine
• ночь — feminine

Стебель is a masculine noun, so it takes:

• крепкий стебель
• он

not:

• крепкая стебель
• она

This is something you usually need to learn with the word.

Here он refers to фикус.

Even though корень and стебель are also masculine singular nouns, the meaning makes it clear that the whole plant is the thing that grows quickly.

So the logic is:

• this ficus has a strong root and strong stem,
• therefore it grows quickly.

Russian pronouns often rely on context, not just on the nearest noun.

Растёт is the 3rd person singular present tense of расти (to grow).

So:

• я расту — I grow / am growing
• ты растёшь
• он/она растёт
• они растут

Here it matches он (the ficus), so растёт is the correct form.

Also note the letter ё. In careful writing, растёт is the standard spelling, although in many texts you may see растет with е instead of ё.

Because быстро is an adverb, and it modifies the verb растёт.

• быстрый = adjective, used with nouns
• быстро = adverb, used with verbs

Compare:

• быстрый рост — fast growth
• он растёт быстро — it grows quickly

So here Russian needs the adverb form.

Yes, here крепкий means something like strong, sturdy, robust, firm.

For plant parts such as a root or stem, крепкий sounds very natural because it suggests good physical solidity and healthy structure.

It is not exactly the same in every context as сильный:

• сильный often means strong/powerful
• крепкий often means sturdy, solid, robust

For a root or stem, крепкий is a very good choice.

Because поэтому introduces the result of the first clause: therefore / so / that's why.

The sentence has two linked parts:

• У этого фикуса крепкий корень и крепкий стебель
• поэтому он растёт быстро

Russian normally separates these clauses with a comma.

So the comma helps show the logic:

strong root and stem → therefore fast growth

Yes, but that would change the meaning.

The singular nouns here present the plant in a general, simple way:

• крепкий корень
• крепкий стебель

This can suggest the plant has a strong root system / main root and a strong stem as a general characteristic.

If you used plurals, you would be talking specifically about multiple roots or stems:

• крепкие корни
• крепкие стебли

So the singular is natural if the speaker is describing the ficus as a type of plant or focusing on its basic structure.