Moppen är bakom dörren, så du behöver inte köpa en ny.

Moppen is the definite singular form of mopp, so it means the mop.

Swedish usually puts the definite article at the end of the noun:

• en mopp = a mop
• moppen = the mop

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

It works the same way as moppen.

• en dörr = a door
• dörren = the door

In Swedish, the definite article is often a suffix attached to the noun, instead of a separate word like English the.

So:

• bakom dörren = behind the door

Bakom means behind.

It is a preposition, so it comes before the thing whose position is being described:

• bakom dörren = behind the door
• bakom huset = behind the house

So Moppen är bakom dörren literally means The mop is behind the door.

Är is the present tense of vara (to be).

It is used here to describe where something is:

• Moppen är bakom dörren = The mop is behind the door

This is the normal way to say that a specific object is located somewhere.

Here så means so or therefore.

It connects the two parts of the sentence:

• Moppen är bakom dörren = The mop is behind the door
• så du behöver inte köpa en ny = so you don’t need to buy a new one

So så shows the result or consequence of the first statement.

In Swedish, inte is the usual word for not.

So:

• du behöver = you need
• du behöver inte = you do not need / you don’t need

This is the normal Swedish way to negate the verb. English uses do not need, but Swedish does not need an extra do.

Also, behöver inte means don’t need to / don’t have to, not must not.

After behöver, Swedish often uses the infinitive without att, especially when behöver works a bit like a modal verb.

So:

• du behöver inte köpa = you don’t need to buy

This is very common and natural Swedish.

En ny means a new one.

The noun mopp is understood and does not need to be repeated. So:

• köpa en ny literally means buy a new [one]
• the missing noun is mopp

This is very common in Swedish, just like in English:

• I need a new one
• Jag behöver en ny

The form of the adjective depends on the noun it refers to.

The hidden noun here is mopp, which is an en-word and singular. So the adjective must match that:

• en ny mopp = a new mop

That is why the shortened version becomes:

• en ny = a new one

Compare:

• en ny bok = a new book
• ett nytt hus = a new house
• nya böcker = new books

Because så here is a coordinating conjunction meaning so, the second clause keeps normal main-clause word order:

• du behöver inte köpa en ny

That means subject + verb:

• du = subject
• behöver = verb

So så du behöver inte köpa en ny is the expected word order here.

Yes. Köpa en ny mopp is completely grammatical.

But Swedish often leaves out the noun when it is obvious from context. Since moppen was already mentioned, en ny is enough.

So these are both possible:

• du behöver inte köpa en ny mopp
• du behöver inte köpa en ny

The second version sounds more natural when the noun is already known.