Если мне хочется чего-нибудь сладкого, я покупаю сырок.

Хочется comes from хотеться, an impersonal verb meaning something like to feel like or to have a desire for.

So:

• я хочу = I want
• мне хочется = I feel like / I have a craving for

The structure is different from English:

• мне is in the dative case
• хочется stays in 3rd person singular neuter form

Literally, it is closer to to me it is wanted or it feels desirable to me, though that literal translation is not natural English.

In this sentence, мне хочется sounds a bit softer and more about a craving or mood than я хочу.

Compare:

• Я хочу кофе. = I want coffee.
• Мне хочется кофе. = I feel like having coffee.

Because хотеться uses the dative for the person who experiences the feeling.

This is common with impersonal expressions in Russian. The person is not the grammatical subject in the usual nominative form.

So:

• мне хочется = I feel like
• тебе хочется = you feel like
• ему хочется = he feels like

This is similar to other Russian patterns such as:

• мне нравится = I like
• мне холодно = I am cold
• мне нужно = I need

In all of these, the experiencer is in the dative.

Because after хочется in this kind of meaning, Russian often uses the genitive to express a desire for some amount or some kind of thing.

So here:

• чего-нибудь is the genitive of что-нибудь
• it means something / anything

You will often see this pattern with food, drink, or vague things you want:

• Хочется чаю. = I feel like some tea.
• Хочется кофе. = I feel like some coffee.
• Хочется чего-нибудь вкусного. = I feel like something tasty.

Using the genitive here gives the sense of some kind of or a bit of something, rather than a specific object.

-нибудь makes the word indefinite and vague. It often means some ... or other, anything, or something depending on context.

So:

• что = what
• что-нибудь = something / anything
• чего-нибудь = of something / something, in the genitive form

In this sentence, чего-нибудь сладкого means something sweet in a non-specific way. The speaker does not have a particular sweet item in mind.

Compare:

• Я хочу торт. = I want a cake. (specific item)
• Мне хочется чего-нибудь сладкого. = I feel like something sweet. (unspecified)

Because it agrees with чего-нибудь, which is genitive.

The phrase is:

• чего-нибудь = genitive
• сладкого = adjective in genitive singular neuter, matching the implied что

So чего-нибудь сладкого literally means something sweet, with sweet describing the indefinite something.

This is a very common pattern in Russian:

• что-нибудь интересное → nominative/accusative
• чего-нибудь интересного → genitive after verbs/structures that call for it

More examples:

• Я купил что-нибудь вкусное. = I bought something tasty.
• Мне хочется чего-нибудь вкусного. = I feel like something tasty.

It is neuter singular because it is linked to что / чего, which is grammatically neuter singular.

Even though in English something sweet does not obviously look neuter, in Russian что is a neuter singular pronoun, and any adjective modifying it matches that form.

So:

• что-нибудь сладкое
• чего-нибудь сладкого

The case changes, but the gender and number stay tied to что.

Yes, если literally means if, but in sentences like this it often has the meaning of whenever or when in a general repeated situation.

So this sentence does not necessarily describe one single future event. It describes a habit or typical reaction:

• Если мне хочется чего-нибудь сладкого, я покупаю сырок.
• If/Whenever I feel like something sweet, I buy a syrok.

Russian often uses если in this kind of general conditional statement.

Compare:

• Если пойдет дождь, мы останемся дома. = If it rains, we’ll stay home.
  one possible future situation

• Если я устаю, я пью чай. = If/Whenever I get tired, I drink tea.
  habitual situation

This sentence is the second type.

Покупаю is the imperfective present tense of покупать. It is used because the sentence describes a habitual action:

• whenever this happens, I buy one

In Russian, the imperfective present is normal for repeated or regular actions.

So:

• я покупаю сырок = I buy a syrok / I usually buy a syrok

If you used куплю, that would be future perfective:

• Если мне захочется чего-нибудь сладкого, я куплю сырок.
• If I feel like something sweet, I’ll buy a syrok.

That would describe a future, more one-time situation.

And покупаюсь is unrelated here; that form would come from покупаться, not покупать.

Сырок is a diminutive form of сыр, but in modern usage it often refers to a specific snack: a small sweet cheese curd bar, often coated in chocolate.

So in this sentence, it usually means that kind of sweet dairy treat, not just a little cheese.

This is useful culturally because a learner might otherwise assume it simply means small cheese. In many everyday contexts, сырок refers to a packaged sweet snack.

Russian uses diminutives very often, and not only to show small size. They can also make a word sound:

• more familiar
• more everyday
• more affectionate
• more like the standard name of an item

In this case, сырок is basically the normal word for that snack. It does not necessarily sound childish or overly cute.

Russian does this a lot with food words and everyday items.

Yes, that is possible, but it is slightly different in tone.

• Если я хочу чего-нибудь сладкого... = If I want something sweet...
• Если мне хочется чего-нибудь сладкого... = If I feel like something sweet / If I’m craving something sweet...

The version with хочется sounds more natural for a passing craving or mood, especially with food.

So the original sentence is a very idiomatic choice.

Yes, the word order can be changed, but the original order is the most neutral.

Russian word order is flexible because case endings show grammatical relationships. Still, different orders create different emphasis.

Neutral:

• Если мне хочется чего-нибудь сладкого, я покупаю сырок.

Possible variation:

• Я покупаю сырок, если мне хочется чего-нибудь сладкого.

This means the same thing, but the focus starts on I buy a syrok.

Inside the clause, мне хочется is also the most natural order. Russian usually keeps these impersonal patterns together.

Because Если мне хочется чего-нибудь сладкого is a subordinate clause introduced by если.

Russian punctuation normally separates the subordinate clause from the main clause with a comma:

• Если X, Y.
• Y, если X.

So the comma here is required.

Yes, extremely common.

Russian speakers often use:

• чего-нибудь вкусного = something tasty
• чего-нибудь горячего = something hot
• чего-нибудь интересного = something interesting
• чего-нибудь выпить = something to drink

This is an important everyday pattern to learn. It is especially common after verbs or expressions of wanting, craving, searching, or needing in a vague, non-specific way.

Yes, but the meaning and structure are a little different.

• что-то сладкое = something sweet
• чего-нибудь сладкого = some kind of sweet thing / something sweet, in a more indefinite, craving-like way

In this sentence, чего-нибудь сладкого sounds especially natural after хочется.

If you say:

• Мне хочется что-то сладкое

many learners may hear or produce this, but native Russian strongly prefers:

• Мне хочется чего-нибудь сладкого
• or Мне хочется чего-то сладкого

So the genitive pattern is the most idiomatic here.