В супермаркете моя тележка была уже полной, поэтому я больше ничего не купил.

Because в can take two different cases depending on the meaning:

• в + accusative = motion into somewhere
  
  • Я иду в супермаркет = I am going to the supermarket
• в + prepositional = location in somewhere
  
  • Я в супермаркете = I am in the supermarket

Here the sentence describes where the speaker was, not movement, so Russian uses the prepositional case: в супермаркете.

Because тележка is a feminine singular noun, so the possessive pronoun must agree with it:

• мой for masculine
• моя for feminine
• моё for neuter
• мои for plural

So:

• мой стол
• моя тележка
• моё место
• мои вещи

Russian adjectives and pronouns usually match the noun in gender, number, and case.

This is a very common question. After forms of быть in the past or future, Russian often uses the instrumental case for the predicate adjective or noun:

• Он был уставшим = He was tired
• Комната была пустой = The room was empty
• Тележка была полной = The cart was full

So полной is the instrumental feminine singular form of полный.

You may also sometimes see была полная, but была полной sounds more standard and natural here. The instrumental is especially common when describing a state or condition.

Yes. That is also possible.

Russian has two main adjective forms here:

• full form: полный / полная / полное / полной, etc.
• short form: полон / полна / полно / полны

So these are both possible:

• тележка была уже полной
• тележка была уже полна

Both mean basically the same thing: the cart was already full.

Very roughly:

• полной sounds a bit more neutral and descriptive
• полна can sound a bit more compact or literary

For a learner, it is good to recognize both.

Уже means already.

So:

• тележка была уже полной = the cart was already full

It shows that by that point, the cart had reached that state, which explains the next clause: поэтому я больше ничего не купил.

Russian уже is used very much like English already:

• Я уже поел = I already ate / I have already eaten
• Уже поздно = It’s already late

Because поэтому means therefore / so / that’s why, and here it connects two clauses:

• В супермаркете моя тележка была уже полной
• поэтому я больше ничего не купил

In Russian, this kind of connector is normally set off by punctuation, so the comma is expected.

Because double negation is normal and required in Russian.

In English, standard grammar says:

• I didn’t buy anything else

But Russian usually says the equivalent of:

• I didn’t buy nothing else

That is not bad grammar in Russian; it is the correct structure.

So:

• ничего = nothing / anything (in negative contexts)
• не купил = did not buy

Together:

• я больше ничего не купил = I didn’t buy anything else / I bought nothing else

This pattern is extremely common:

• Я никого не видел = I didn’t see anyone
• Он никогда не опаздывает = He never arrives late
• Мы нигде не были = We weren’t anywhere

Ничего is the form used here of ничто / nothing.

In this sentence, it appears in the form ничего, which is used after negation and is the normal form in expressions like:

• ничего не знаю = I don’t know anything
• ничего не вижу = I don’t see anything
• ничего не купил = I didn’t buy anything

For learners, the most important thing is to treat ничего не + verb as a very common pattern meaning not ... anything or ... nothing.

Also, with больше, the phrase больше ничего means nothing more / nothing else / anything else.

Here больше means any more / anything more / anything else.

So:

• я больше ничего не купил = I didn’t buy anything else

This is a very common use of больше in negative sentences:

• Я больше не курю = I don’t smoke anymore
• Он больше не придёт = He won’t come anymore
• Мы больше ничего не заказывали = We didn’t order anything else

So although больше literally relates to more, in context it often translates more naturally as any more or else.

Because the sentence talks about a completed result: in the end, the speaker did not buy any more items.

• купить = perfective, to buy / to complete a purchase
• покупать = imperfective, to be buying / to buy repeatedly or in process

Here the speaker means: after the cart was full, no further purchases happened. That is a completed fact, so купил is natural.

Compare:

• Я ничего не купил = I didn’t buy anything
• Я ничего не покупал = I wasn’t buying anything / I didn’t do any buying

The perfective купил fits best when talking about the final outcome.

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

This sentence is natural as written:

• В супермаркете моя тележка была уже полной, поэтому я больше ничего не купил.

But you could also hear variations such as:

• В супермаркете моя тележка уже была полной
• Поэтому я ничего больше не купил

The basic meaning stays the same, but the focus changes slightly:

• уже была полной may emphasize already
• ничего больше may sound slightly more neutral than больше ничего
• putting something earlier often gives it more emphasis

For learners, the original sentence is perfectly natural and worth imitating.

Тележка literally means a small cart or trolley. In a supermarket context, it means a shopping cart / trolley.

So in this sentence, моя тележка clearly refers to the cart the speaker is pushing around the store.

A related word is:

• корзина = basket

So:

• тележка = cart/trolley
• корзина = basket

If the speaker had only a handheld basket, the sentence would probably use корзина instead.