Утром я купил творожный сырок в магазине у метро.

Утром is the instrumental singular of утро (morning), and Russian often uses this form adverbially to mean in the morning.

So:

• утро = morning as a noun
• утром = in the morning as a time expression

This is very common with parts of the day:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So here утром is not an object; it tells you when the action happened.

Купить / купил is perfective, so it presents the action as completed: the speaker bought the item successfully.

In this sentence, that makes sense because it describes one finished event in the morning.

Compare:

• я купил = I bought / I did buy
• я покупал = I was buying / I used to buy / I bought (process-focused, repeated, or backgrounded)

If you are simply reporting a completed purchase, купил is the natural choice.

Yes. In the past tense, Russian verbs show gender and number.

• купил = masculine singular
• купила = feminine singular
• купило = neuter singular
• купили = plural

So я купил tells us the speaker is male. If the speaker were female, it would be я купила.

This is one reason Russian often keeps the subject pronoun я in the past tense: the verb shows gender, but not person as clearly as present-tense forms do.

This is a specific food item. It usually means a small curd-cheese snack/bar, often sweet, sometimes chocolate-coated.

Breaking it down:

• творожный = made from творог (curd cheese / farmer cheese / quark-like dairy product)
• сырок = literally a small cheese, from сыр with a diminutive ending

But as a phrase, творожный сырок should be understood as a set food name, not just any random small cheese.

Because it is a masculine inanimate singular direct object.

In Russian, for masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: творожный сырок
• accusative: творожный сырок

That is why after купил you still see творожный сырок, not a different-looking ending.

Compare with an animate noun:

• я вижу нового студента
  The adjective and noun change because студент is animate.

Because this sentence describes location, not movement toward a destination.

• в магазине = in the store / at the store
• в магазин = to the store

Russian uses:

• в + prepositional for being somewhere
• в + accusative for going into/toward somewhere

So here:

• купил в магазине = bought it in/at the store
• пошёл в магазин = went to the store

У метро means by the metro, near the metro, or next to the metro.

The preposition у commonly means near/by when talking about location.

So:

• магазин у метро = a store near the metro / near the subway station

It does not mean that the store belongs to the metro. It just tells you where the store is located.

Normally, у takes the genitive case. However, метро is one of those Russian nouns that are indeclinable, meaning its form stays the same in all cases.

So even though grammatically it is functioning after у, the word still appears as:

• метро

This is very common with some borrowed words in Russian.

Yes, Russian can often omit subject pronouns, but whether that sounds natural depends on context.

Here, я is perfectly normal. It makes the sentence clear and straightforward.

In the past tense, the verb does not show person as clearly as present-tense verbs do, so the pronoun is often useful:

• купил by itself means bought and shows masculine singular, but not explicitly I versus he without context
• я купил clearly means I bought

If the context already made the subject obvious, Russian could omit я.

Yes, it could be rearranged. Russian word order is fairly flexible.

This version:

• Утром я купил творожный сырок в магазине у метро.

is a natural neutral way to present the information: time → subject → verb → object → place

But other orders are possible depending on emphasis:

• Я утром купил творожный сырок в магазине у метро.
• Творожный сырок я купил утром в магазине у метро.
• В магазине у метро я утром купил творожный сырок.

The basic meaning stays the same, but the focus changes.

Sometimes, yes, but the nuance is different.

• утром = in the morning
• с утра = since morning / from the morning on, or sometimes early in the morning

So in this sentence, утром is the most neutral and direct choice.

If you said С утра я купил..., it might sound a bit more marked and could suggest a different emphasis, depending on context. For a simple statement of time, утром is best.