Аптека находится на противоположной стороне улицы.

Находится means is located / is situated. In Russian, this is a very common way to talk about where something is.

• Аптека находится... = The pharmacy is located...
• In the present tense, Russian usually does not use есть to mean simple is/are.
• You could omit the verb in some short, conversational contexts, but here находится sounds natural and clear.

So:

• Аптека на противоположной стороне улицы = possible in casual speech
• Аптека находится на противоположной стороне улицы = fuller, standard, very natural

Стороне is in the prepositional case.

The preposition на can take different cases, but when it means location (on / at), it usually takes the prepositional:

• на столе = on the table
• на улице = in the street / outside
• на стороне = on the side

Here the pharmacy is located somewhere, so we use the location pattern:

• на противоположной стороне

That is why сторона becomes стороне.

Because противоположной is an adjective agreeing with стороне.

The noun сторона is:

• feminine
• singular
• here in the prepositional case

So the adjective must match it:

• nominative: противоположная сторона
• prepositional: на противоположной стороне

This is normal Russian adjective agreement.

Because Russian often uses the genitive after one noun to show a relationship like of.

So:

• сторона улицы = the side of the street

This is the same idea as:

• центр города = the center of the city
• дверь дома = the door of the house

In the sentence, the full phrase is:

• на противоположной стороне улицы

Literally: on the opposite side of the street

So улицы is genitive singular of улица.

Literally, it breaks down like this:

• на = on
• противоположной = opposite
• стороне = side
• улицы = of the street

So the literal structure is:

on the opposite side of the street

This matches English quite closely, although English often also says across the street.

Yes, often it could.

• Аптека находится через улицу.
• Аптека находится на противоположной стороне улицы.

Both can mean something like The pharmacy is across the street.

The difference is mostly one of phrasing:

• на противоположной стороне улицы is a bit more explicit and literally describes the location
• через улицу is often a bit more direct and idiomatic for across the street

In many everyday situations, they are very close in meaning.

Because на стороне here means on the side in a physical, spatial sense.

• на стороне улицы = on the side of the street

But в стороне often means something more like:

• aside
• off to the side
• away from the main area

For example:

• Дом стоит в стороне от дороги. = The house stands off to the side of the road.

So in this sentence, на стороне улицы is the correct choice because we are talking about one side of the street itself.

Yes. Russian word order is flexible.

The original sentence is:

• Аптека находится на противоположной стороне улицы.

You could also say:

• На противоположной стороне улицы находится аптека.

Both are correct.

The difference is in emphasis:

• Аптека находится... starts with the pharmacy
• На противоположной стороне улицы... starts with the location

The first version is very neutral. The second can sound a little more descriptive or can emphasize where it is.

Usually, yes, it is the normal word for pharmacy.

Depending on the variety of English, it may also feel close to:

• chemist’s (British English)
• drugstore (American English)

But аптека is specifically the place where medicines are sold. It does not automatically carry all the broader meanings that drugstore can sometimes have in American English.

So pharmacy is usually the safest translation.

A helpful stressed version is:

Аптека́ нахо́дится на противополо́жной стороне́ у́лицы.

Main stress points:

• аптека́
• нахо́дится
• противополо́жной
• стороне́
• у́лицы

A rough pronunciation guide:

• аптека = ap-tye-KA
• находится = na-KHO-dit-sya
• противоположной = pra-ti-va-pa-LOZH-nay
• стороне = sta-ra-NYE
• улицы = OO-li-tsy

If you want to sound natural, try not to stress every word equally; Russian has a clear rhythm with one main stress in each word.