Hon tappade musen på golvet, men tangentbordet fungerar fortfarande.

Because musen is the definite form, meaning the mouse. The basic form is mus = a mouse. In Swedish, definiteness is usually added as an ending:

• en mus = a mouse
• musen = the mouse

So Hon tappade musen means She dropped the mouse, not just She dropped a mouse.

Swedish usually puts the at the end of the noun instead of using a separate word before it.

Here are the base forms:

• en mus → musen = the mouse
• ett golv → golvet = the floor
• ett tangentbord → tangentbordet = the keyboard

So English uses a separate word, but Swedish often uses a suffix.

That depends on the gender of the noun.

Swedish nouns are mainly divided into two groups:

• en-words
• ett-words

Mus is an en-word:

• en mus
• musen

Golv and tangentbord are ett-words:

• ett golv

• golvet

• ett tangentbord
• tangentbordet

So the definite ending often matches the noun type:

• en-word → usually -en
• ett-word → usually -et

Tappade is the past tense of tappa. In this sentence, it means dropped or accidentally let fall.

That is slightly different from a verb like släppte, which often means let go of or released.
So Hon tappade musen suggests that she dropped the mouse by accident.

Because the sentence describes two different time relationships:

• Hon tappade musen = an action that happened in the past
• men tangentbordet fungerar fortfarande = the keyboard still works now

So the sentence is saying: she dropped the mouse earlier, but the keyboard is still functioning at the present time.

If it were all in the past, you might see something like fungerade fortfarande, meaning was still working.

Fortfarande means still.

In this sentence:

• tangentbordet fungerar fortfarande = the keyboard still works / the keyboard is still working

It shows that something continues to be true even after what happened before.

That is a normal Swedish word order for this kind of sentence. In a main clause, the finite verb usually comes early, and adverbs like fortfarande often come after it.

So:

• tangentbordet fungerar fortfarande

is the natural order.

English often does something similar:

• the keyboard still works but Swedish commonly uses:
• subject + verb + adverb

På golvet means on the floor.

• på = on
• golvet = the floor

This is the normal expression for something ending up on the floor. A native English speaker might expect something more literal like to the floor, but Swedish naturally uses på here.

Because men is a coordinating conjunction, meaning it joins two main clauses of equal status.

So the second clause keeps normal main-clause word order:

• men tangentbordet fungerar fortfarande

That is:

• men
  • subject + verb + adverb

Swedish does inversion after certain sentence-opening elements, but men itself does not force that here.

In this sentence, it clearly means a computer mouse because of the context: tangentbordet means the keyboard.

The word mus can mean both:

• a mouse, the animal
• a computer mouse

Swedish uses the same word for both, just like English uses mouse in both senses.

The dictionary forms are:

• hon = she
• tappa = to drop
• mus = mouse
• golv = floor
• men = but
• tangentbord = keyboard
• fungera = to function / to work
• fortfarande = still

In the sentence, some of these appear in changed forms:

• tappade = past tense of tappa
• musen = definite form of mus
• golvet = definite form of golv
• tangentbordet = definite form of tangentbord
• fungerar = present tense of fungera

A more indefinite version would be something like:

Hon tappade en mus på ett golv, men ett tangentbord fungerar fortfarande.

But that sounds unnatural in most real situations, because the sentence is about specific objects: the mouse, the floor, and the keyboard.

So the definite forms in the original sentence are the natural choice:

Hon tappade musen på golvet, men tangentbordet fungerar fortfarande.