Её тележка была почти полной, а продавщица поставила сыр на весы.

Её is the possessive form meaning her.

A useful thing to know is that её can mean either:

• her as a possessive: её тележка = her cart
• her as an object: я вижу её = I see her

In её тележка, it must mean her because it comes directly before a noun.

Also, unlike many other Russian words, possessive её does not change form for gender, number, or case:

• её тележка = her cart
• её сумка = her bag
• её дети = her children

That makes it easier to use than many adjectives or pronouns.

Тележка literally means small cart or trolley, but in this context it means a shopping cart.

It comes from телега or related cart words, and the -к- / -жк- type ending often gives a smaller or more everyday sense. In modern usage, тележка is the normal word for a shopping cart in a store.

So here:

• её тележка = her shopping cart

This is a very common learner question.

After быть in the past tense, Russian often uses the instrumental case for a predicate noun or adjective-like description. That is why you get:

• тележка была полной = the cart was full

Here, полной is the instrumental feminine singular form of полная.

So:

• nominative feminine: полная
• instrumental feminine: полной

With predicates, Russian has a few possibilities, and they do not always feel exactly the same:

• тележка была полной — very normal, standard
• тележка была полна — also correct, using the short form adjective
• тележка была полная — possible in some contexts, but less neutral here

So была почти полной is a standard, natural way to say was almost full.

Почти means almost. It is an adverb, so it modifies the adjective полной.

Structure:

• почти полной = almost full

This works like English:

• almost full
• almost ready
• almost empty

In Russian:

• почти готова = almost ready
• почти пустая = almost empty
• почти полной = almost full

So почти does not change form. It just adds the meaning almost.

А often means something like:

• and with a contrast
• while
• whereas
• a shift from one part of the scene to another

In this sentence, а links two related facts but also shifts attention:

• Her cart was almost full
• and meanwhile / while that was the case, the saleswoman put the cheese on the scales

So а feels more natural than и because it moves the listener from one image to the next.

Very roughly:

• и = simple and
• а = and / while / whereas with a slight contrast or change of focus

Yes.

• продавец = salesperson, seller
• продавщица = female salesperson, saleswoman

Since the verb is also feminine:

• поставила

we know the person is female, so продавщица fits naturally.

A small usage note:

• продавец can be used in a more general or job-title sense
• продавщица is specifically feminine and often feels more everyday

In this sentence, продавщица simply means the saleswoman.

In the past tense, Russian verbs agree in gender and number with the subject.

The subject here is продавщица, which is feminine singular, so the verb must also be feminine singular:

• masculine: поставил
• feminine: поставила
• neuter: поставило
• plural: поставили

So:

• продавщица поставила = the saleswoman put

This is one of the key features of the Russian past tense.

This is about aspect.

• ставить = imperfective
• поставить = perfective

Here, the sentence describes a completed action: she put the cheese on the scales. Because the action is viewed as a single finished event, Russian uses the perfective verb:

• поставила

If you said ставила, it would suggest something more like:

• she was putting it down
• she used to put it down
• the action is being viewed as process/repetition rather than a completed event

So поставила is the natural choice for a one-time completed action in a narrative.

It is in the accusative case, but for inanimate masculine singular nouns, the accusative usually looks exactly like the nominative.

So:

• nominative: сыр
• accusative: сыр

That is why you do not see a visible change.

Compare:

• Я вижу сыр = I see cheese
• Я купил сыр = I bought cheese

But with an animate masculine noun, accusative often matches genitive instead:

• Я вижу брата = I see my brother

So сыр is accusative here, even though it looks unchanged.

This is a case-choice question after на.

Russian uses:

• на + accusative for movement onto a surface or destination
• на + prepositional for location on a surface

Here the cheese is being moved onto the scales, so Russian uses на + accusative:

• поставила сыр на весы = put the cheese onto the scales

If you were describing where it already is, you would use:

• сыр на весах = the cheese is on the scales

So the contrast is:

• на весы = onto the scales
• на весах = on the scales

Yes, весы is normally a plural-only noun in Russian when it means scales.

So you say:

• эти весы = these scales
• на весах = on the scales
• к весам = toward the scales

There is no normal everyday singular form for this meaning.

English is actually similar in a way, because scales is also usually plural when it means a weighing device.

Yes. Russian word order is more flexible than English word order, because case endings show the grammatical roles.

The sentence as written is neutral and natural:

• Её тележка была почти полной, а продавщица поставила сыр на весы.

But Russian could rearrange parts for emphasis. For example:

• Сыр продавщица поставила на весы — puts extra focus on сыр
• На весы продавщица поставила сыр — focuses on the destination, onto the scales

Even so, the original order is probably the most straightforward for a learner and sounds very natural in a normal narrative.