Если добавить в бульон маленькие фрикадельки и сельдерей, суп становится сытнее.

Russian often uses an impersonal infinitive in general statements, especially in recipes and instructions.

So Если добавить... means something like:

• if you add...
• if one adds...
• if we add...

The person doing the action is left unspecified because it is not important.

Добавить is perfective, so it presents the action as a completed addition that produces a result.

Here the idea is:

• first, the ingredients get added
• then, as a result, the soup becomes more filling

That is why добавить fits well.

If you used добавлять, it would sound more like an ongoing, repeated, or habitual process of adding, which is less natural in this sentence.

Because в can take different cases depending on meaning:

• в + accusative = motion/direction into something
• в + prepositional = location inside something

Here the ingredients are being put into the broth, so Russian uses в бульон.

Compare:

• добавить в бульон — add into the broth
• вариться в бульоне — cook in the broth

It is accusative plural, because it is a direct object of добавить.

Why does it look like nominative plural? Because фрикадельки are inanimate, and for inanimate plural nouns, accusative = nominative.

So:

• nominative plural: маленькие фрикадельки
• accusative plural: маленькие фрикадельки

The adjective маленькие matches the noun.

It is also accusative, because it is another direct object of добавить.

But сельдерей is a masculine inanimate noun, and in the singular, accusative often looks the same as nominative:

• nominative: сельдерей
• accusative: сельдерей

So the form does not change.

Because the sentence begins with an если clause:

• Если добавить в бульон маленькие фрикадельки и сельдерей, ...

This is a subordinate clause, and in Russian it is normally separated from the main clause by a comma.

So the structure is:

• Если ... , суп становится сытнее.

Russian can say both:

• Если ..., суп становится сытнее.
• Если ..., то суп становится сытнее.

The word то is optional here. It can make the cause-and-result relationship a little more explicit, but leaving it out is very normal and natural.

Становится is present tense, and Russian often uses the present tense for general truths or regular results.

So the sentence means something like:

• whenever you add these ingredients, the soup becomes more filling

If you said станет, it would sound more like a specific future result in one particular case:

• add them now, and the soup will become more filling

Both are possible in the right context, but становится is better for a general statement.

Сытнее is the comparative of сытный.

• сытный — filling, hearty
• сытнее — more filling, heartier

Russian comparatives like сытнее do not change for gender, number, or case. So even though суп is masculine singular, the comparative stays just сытнее.

That is why Russian says:

• суп становится сытнее

not something with masculine agreement on the comparative.

Yes. Russian word order is flexible.

For example, you could also say:

• Суп становится сытнее, если добавить в бульон маленькие фрикадельки и сельдерей.

The meaning stays basically the same. The original version is natural because it puts the condition first.

This reflects a natural cooking idea:

• бульон = the broth/base liquid
• суп = the finished soup

So the sentence starts from the idea of adding ingredients to the broth, and then talks about the result as the soup becoming more filling.

In other words, Russian is viewing the dish first as a broth base, then as the soup you end up with.