На ужин я запеку брокколи с сыром.

Because these are two different meanings.

• на ужин = for dinner
• на ужине = at dinner / at the dinner event

In this sentence, на ужин tells us what the food is intended for. It is a very common pattern in Russian:

• на завтрак = for breakfast
• на обед = for lunch
• на ужин = for dinner

So На ужин я запеку... means For dinner, I’ll bake..., not While at dinner...

It actually is in the accusative case, but for a masculine inanimate noun like ужин, the accusative singular looks exactly like the nominative singular.

So:

• nominative: ужин
• accusative: ужин

That is why you do not see a visible ending change.

Запеку is the 1st person singular future form of the verb запечь.

So it means:

• я запеку = I will bake / I’ll roast

A useful breakdown:

• за- = a prefix
• печь / запечь = to bake
• -у = the ending for I in this future form

Because запечь is a perfective verb, its future is expressed with one simple form: запеку.

This is the classic Russian perfective vs. imperfective distinction.

• запеку from запечь = perfective
  Focuses on a single completed result: I’ll bake it
• буду запекать from запекать = imperfective
  Focuses more on the process, duration, or repeated action: I’ll be baking / I’ll bake (in a more process-oriented sense)

In this sentence, the speaker is talking about a specific dinner plan and the finished dish, so запеку is the natural choice.

Not always. Russian often omits subject pronouns when they are clear from the verb ending.

So both are possible:

• На ужин я запеку брокколи с сыром.
• На ужин запеку брокколи с сыром.

The version with я can sound a little clearer, a little more explicit, or slightly contrastive depending on context. In everyday speech, both are natural.

Here брокколи is the direct object, so it is in the accusative case.

However, брокколи is an indeclinable noun in standard Russian, so its form stays the same. You do not add endings to it.

So:

• nominative: брокколи
• accusative: брокколи
• and usually the same in other cases too

That is why it looks unchanged.

In Russian, брокколи is normally treated as a singular noun, even though in English learners may associate it with a mass noun or with multiple florets.

In dictionaries, it is usually listed as an indeclinable feminine singular noun. So in this sentence, it means broccoli as a dish or ingredient, not broccolis.

Because the preposition с meaning with normally requires the instrumental case.

So:

• сыр = cheese
• с сыром = with cheese

That -ом ending is the instrumental singular ending for this masculine noun.

This is a very common pattern:

• с другом = with a friend
• с супом = with soup
• с сыром = with cheese

Yes, that is the natural reading.

Запечь брокколи с сыром means to bake broccoli with cheese as one dish. The phrase с сыром describes what the broccoli is combined with.

So the sentence suggests something like a baked broccoli-and-cheese dish, not two totally separate items.

Yes. Russian word order is flexible, though different orders shift emphasis.

The original:

• На ужин я запеку брокколи с сыром.

This puts На ужин first, which sets the topic: as for dinner...

Other natural possibilities:

• Я запеку на ужин брокколи с сыром.
• Брокколи с сыром я запеку на ужин.

These all mean roughly the same thing, but the focus changes:

• sentence-initial На ужин emphasizes for dinner
• sentence-initial Брокколи с сыром emphasizes what is being made

Not exactly.

• печь = to bake, to roast; the basic verb, usually imperfective
• испечь = to bake and produce something by baking, often things like bread, pies, cakes
• запечь = to bake/roast something, often in the oven, sometimes with the idea of cooking it through or giving it a browned top

For food like брокколи с сыром, запечь is very natural. It sounds like putting the dish into the oven and baking it as a prepared dish.

Russian has no articles, so there is no separate word for the or a/an.

That means брокколи с сыром can be interpreted from context as:

• broccoli with cheese
• the broccoli with cheese
• sometimes even some broccoli with cheese

English has to choose an article if needed, but Russian usually leaves that to context.