Apoteket ligger på andra sidan gatan.

Because apoteket is the definite form, meaning the pharmacy.

In Swedish, the definite article is usually added to the end of the noun:

• ett apotek = a pharmacy
• apoteket = the pharmacy

So the sentence is talking about a specific pharmacy, not just any pharmacy.

Apotek is a neuter noun in Swedish, so it takes ett in the indefinite form:

• ett apotek = a pharmacy

That is why the definite ending is -et:

• apoteket = the pharmacy

Yes, ligger literally means lies, but in Swedish it is very commonly used to describe where places and buildings are located.

So:

• Apoteket ligger ... = The pharmacy is located ...

This is more natural in Swedish than using är here.

Compare:

• Huset ligger nära stationen. = The house is near the station.
• Skolan ligger i centrum. = The school is in the center.

So when talking about the location of a building, shop, town, island, etc., ligger is extremely common.

You might be understood, but ligger is much more natural when talking about the physical location of a place or building.

• Apoteket ligger på andra sidan gatan. = natural
• Apoteket är på andra sidan gatan. = understandable, but less idiomatic in this context

Swedish often prefers ligger for fixed location.

It means on the other side of the street.

The phrase breaks down like this:

• på = on
• andra = other / second
• sidan = the side
• gatan = the street

So literally it is something like on the other side the street, but in natural English we say on the other side of the street or simply across the street.

In this sentence, på andra sidan is a very common fixed expression meaning on the other side.

Swedish often omits den in this kind of phrase. So:

• på andra sidan gatan = on the other side of the street

You may also see fuller versions in other contexts, such as:

• på den andra sidan av gatan

That is also possible, but på andra sidan gatan is very common and natural.

Because Swedish often leaves out av in this expression.

So both of these can occur:

• på andra sidan gatan
• på andra sidan av gatan

The version without av is very common and idiomatic.

English requires of in the other side of the street, but Swedish does not always need av here.

Because gatan is the definite form of gata:

• en gata = a street
• gatan = the street

In this expression, Swedish normally uses the definite form:

• på andra sidan gatan = on the other side of the street

This works much like English, where we also usually say the street, not a street, in this kind of sentence.

Yes, in most everyday situations they mean the same thing.

• Apoteket ligger på andra sidan gatan.
• Natural English: The pharmacy is across the street.

A more literal English translation would be:

• The pharmacy is on the other side of the street.

So across the street is usually the best natural translation.

Usually, på andra sidan gatan is the safer and more natural choice for location.

Över gatan often suggests movement across the street rather than location on the other side.

For example:

• Jag går över gatan. = I am walking across the street.

But for where something is located, Swedish usually prefers:

• Apoteket ligger på andra sidan gatan.

So if you mean a place is located across the street, use på andra sidan gatan.

The sentence follows normal Swedish word order:

• Apoteket = subject
• ligger = verb
• på andra sidan gatan = place expression

So:

Subject + verb + place

This is the same basic pattern as in simple English sentences:

• The pharmacy
  • is located
    • across the street

A rough guide for an English speaker is:

ah-poh-TEK-et LIG-er paw AN-dra SEE-dan GAH-tan

A few helpful notes:

• Apoteket has the main stress on tek
• ligger has a short i, roughly like i in sit
• på sounds like paw
• gatan begins with a hard g

Pronunciation varies a bit by region, but this will get you close.

It is completely neutral and natural in both speech and writing.

You could use it:

• in conversation
• when asking for directions
• in written instructions
• in everyday descriptions of location

So it is neither especially formal nor especially informal. It is just standard Swedish.