Learn Hardcore Russian: Когда я сижу на диете, я не пью сладкую газировку. - When I am on a diet, I do not drink sweet soda.

Literally: сидеть = “to sit,” на диете = “on a diet.”
Idiomatically: it describes being in a state of following a diet (usually for some time, with effort or restriction).

You can also say:

Когда я на диете, я не пью сладкую газировку. – also correct and common.

The version with сижу is a bit more “vivid” and colloquial, suggesting you’re “stuck” in this diet state, similar to Russian idioms like:

сидеть в тюрьме – to be in prison
сидеть на игле – to be addicted (literally “sit on the needle”)

So сижу на диете is just a standard idiom, not about physically sitting.

Why is the present tense (сижу, пью) used if this is about a general rule (“when I’m on a diet, I don’t drink…”) and not about right now?

Russian, like English, uses the present tense to talk about general habits and rules.

Когда я сижу на диете, я не пью сладкую газировку.
= “Whenever I’m on a diet, I don’t drink sweet soda.”

This is the same kind of present as in English:

“When I drive, I wear a seatbelt.”
“When I’m on a diet, I don’t drink soda.”

So сижу (“I sit / I am on”) and пью (“I drink / I am drinking”) are present tense, imperfective, and they naturally express repeated, habitual actions in Russian.

Why is there a comma between Когда я сижу на диете and я не пью сладкую газировку?

In Russian, a clause introduced by когда (“when”) is a subordinate clause. The rule is:

Separate the subordinate clause from the main clause with a comma.

So you get:

Когда я сижу на диете, – subordinate (time condition)
я не пью сладкую газировку. – main clause

You can also reverse the order:

Я не пью сладкую газировку, когда я сижу на диете.

The meaning is the same; only the information flow and intonation change slightly. The comma remains in both word orders.

Could you replace когда with если here? What’s the difference between когда and если in this sentence?

You can say both, but there’s a nuance:

Когда я сижу на диете, я не пью сладкую газировку.
– Neutral, factual: whenever / when(ever) I’m on a diet, I don’t drink sweet soda.
It implies this situation (“me being on a diet”) actually happens from time to time.
Если я сижу на диете, я не пью сладкую газировку.
– More conditional: if I’m on a diet, I don’t drink sweet soda.
It focuses more on the condition being fulfilled, possibly hypothetical.

In everyday speech, they often overlap, and both would be understood similarly here, but когда sounds more like a description of a regular pattern.

Why do we use на in на диете and not some other preposition like в?

The combination на + prepositional is simply the standard pattern for this meaning:

быть на диете / сидеть на диете – to be on a diet

Some states and activities in Russian are expressed with на:

на работе – at work
на лекции – at the lecture
на уроке – in class
на посту – on duty

Here, на диете treats “diet” as a sort of state / regime / program you’re “on.”
В диете would sound incorrect in this context.

Grammatically, диета (feminine) in the prepositional case becomes диете:

Nom.: диета
Prep.: на диете

Why is it сладкую газировку and not сладкая газировка?

Because in “я не пью сладкую газировку”, сладкую газировку is the direct object of the verb пью (“I drink”).

Direct objects in Russian usually take the accusative case. For a feminine noun ending in -а, the singular accusative ending is -у:

Nominative: сладкая газировка – sweet soda (as subject)
Accusative: сладкую газировку – sweet soda (as object)

So:

Сладкая газировка вредна. – Sweet soda is unhealthy. (subject → nominative)
Я не пью сладкую газировку. – I don’t drink sweet soda. (object → accusative)

The adjective сладкую agrees with the noun газировку in gender, number, and case:
feminine, singular, accusative.

Why is газировку ending with -у, and what case is it?

Газировка is a feminine noun of the -а type:

Nom. sg.: газировка
Acc. sg.: газировку

In this sentence, it’s the direct object of пью, so it’s in the accusative case:

(Я) пью газировку. – I drink soda.

For inanimate feminine nouns ending in -а, accusative = nominative form but with -у ending:

книга → книгу (a book)
машина → машину (a car)
газировка → газировку (soda)

Could you explain the difference between я не пью сладкую газировку and я пью не сладкую газировку?

Yes, the position of не changes the meaning:

Я не пью сладкую газировку.
- I don’t drink sweet soda (at all).
- не negates the whole verb phrase “drink sweet soda.”
Я пью не сладкую газировку.
- I drink not sweet soda (but some other kind: maybe diet soda, mineral water, etc.).
- не directly negates the adjective сладкую: the soda I drink is specifically not sweet.

So:

Sentence 1 = total refusal of sweet soda.
Sentence 2 = you do drink soda, but it is not sweet.

In the original sentence, the intended meaning is clearly the first one.

Why is it я не пью сладкую газировку and not я не пью сладкой газировки? I thought negation often uses the genitive case.

Both accusative and genitive can appear after не with a direct object, but they give slightly different nuances.

Accusative after negation – normal, neutral, total negation:
- Я не пью сладкую газировку.
  = I don’t drink sweet soda (as a rule, basically never).
Genitive after negation – can imply absence, lack, or “not any / not even a bit”, often a bit more emotional or demonstrative:
- Я не пью сладкой газировки.
  = I don’t drink (any) sweet soda at all / I don’t touch sweet soda.

In modern spoken Russian, accusative after negation is very common and completely correct. The genitive tends to sound either more emphatic, more “bookish,” or sometimes old-fashioned, depending on context.

So the textbook-like, neutral choice here is exactly what you see: не пью сладкую газировку.

Can I drop the pronoun я and say Когда сижу на диете, не пью сладкую газировку?

Yes, that’s perfectly natural in Russian:

Когда сижу на диете, не пью сладкую газировку.

Russian often omits subject pronouns if the subject is clear from the verb ending. The verb forms сижу and пью already tell us that the subject is “I” (1st person singular).

Including я adds a bit of explicitness or emphasis on “I”:

Когда я сижу на диете, я не пью…

Omitting it sounds a bit more compact and conversational. Both versions are grammatically correct.

Is there any difference between когда я сижу на диете, я не пью… and когда я на диете, я не пью…?

Both are correct and very common, with a slight nuance:

Когда я на диете, я не пью сладкую газировку.
– Literally: “When I am on a diet…”
– A bit more neutral, simply states your status.
Когда я сижу на диете, я не пью сладкую газировку.
– Literally: “When I sit on a diet…”
– Idiomatic, slightly more colloquial / expressive, can hint at the idea of enduring or following the diet regime.

In many situations they are interchangeable, and most natives wouldn’t feel a big difference.

How would I say this if I wanted to talk about a single specific period of diet, not a general habit? Would the verb aspect change?

If you’re talking about one specific diet period in the past, you’d normally switch to past tense, still with imperfective aspect for ongoing/habitual behavior during that period:

Когда я сидел(а) на диете, я не пил(а) сладкую газировку.
– When I was on a diet (that time), I didn’t drink sweet soda.

Aspect doesn’t usually change here (пить imperfective is fine), because you describe a repeated/ongoing restriction during that time. A perfective verb выпить (“to drink up / to have a drink (once)”) would be used if you were talking about one-time completed actions, e.g.:

Когда я сидел(а) на диете, я ни разу не выпил(а) сладкой газировки.
– While I was on a diet, I didn’t drink sweet soda even once.

In the original sentence (general rule), the present imperfective сижу, пью is exactly what you want.

What exactly does газировка mean? Is it “soda,” “soft drink,” “sparkling water,” or something else?

Газировка is a somewhat informal, everyday word. Depending on context, it can mean:

Carbonated soft drink / soda (often sweet, flavored)
Historically, it could also mean just carbonated water (especially from street machines in the USSR).

In modern usage, if you say сладкая газировка, people usually think of sweet fizzy soft drinks like Coke, Fanta, Sprite, etc.

If you specifically mean plain sparkling water, you’d more often hear:

газированная вода
минеральная вода с газом – sparkling mineral water

So in this sentence, сладкая газировка is best translated as “sweet soda / sugary soft drinks.”

Where is the stress in the words сижу, диете, пью, сладкую, газировку?

Stress in these words is as follows (stressed syllable in caps):

сиЖУ
диЕте
ПЬЮ (one syllable; the whole word is stressed)
СЛАДкую (stress stays on the root: сЛАдкую)
газиРОвку

So pronounced together:

Когда я сиЖУ на диЕте, я не ПЬЮ СЛАдкую газиРОвку.

Paying attention to stress is important in Russian because it can affect both pronunciation and, in other words, sometimes meaning.