Myggen sidder på min arm.

Because myggen is the definite singular form.

• en myg = a mosquito
• myggen = the mosquito

In Danish, the idea of English the is often added as an ending on the noun instead of as a separate word.

The basic noun is myg, and the definite ending is -en. When that ending is added, Danish spelling often doubles the final consonant after a short stressed vowel:

• myg → myggen

The extra g is a spelling feature. It helps show that the vowel stays short.

Sidder is the present tense of at sidde, which means to sit.

So:

• at sidde = to sit
• sidder = sits, is sitting

If you wanted the past tense, it would be sad.

Danish very often uses position verbs such as sidde, stå, and ligge when describing where something is.

So instead of just saying that the mosquito is on your arm, Danish naturally says it sits on your arm.

That does not mean the mosquito is sitting like a person in a chair. It just means it is resting/positioned there.

A sentence with er could be understood, but sidder på min arm sounds much more natural and idiomatic.

Not necessarily.

Sidder only tells you that the mosquito is on your arm, settled there. It may be about to bite, already biting, or simply resting there. The verb itself does not specify that.

Because på means on, and the mosquito is on the surface of your arm.

That is the natural choice here.

Compare:

• på = on
• i = in, inside
• ved = by, near

So på min arm is exactly what you would expect for something physically on your arm.

Because arm is a common-gender noun in Danish:

• en arm

With common-gender singular nouns, the possessive is min:

• min arm = my arm

Compare:

• min for singular en-words
• mit for singular et-words
• mine for plural nouns

So mit arm would be wrong here.

Because in Danish, a possessive like min already does the job that an article would do.

So Danish says:

• min arm = my arm

not something like the my arm.

This works the same way as in English.

Yes. This sentence has normal main-clause word order:

• Myggen = subject
• sidder = finite verb
• på min arm = prepositional phrase

So the pattern is basically:

• Subject + verb + rest

More generally, Danish follows the verb-second rule in main clauses. That means the finite verb normally comes in the second position.

For example, you could also say:

• På min arm sidder myggen

That is still correct, but now på min arm comes first for emphasis, and the verb sidder still stays in second position.

Yes, sometimes.

• på min arm = on my arm
• på armen = on the arm

If the context already makes it clear whose arm it is, Danish can use the definite form armen. But min arm is clearer when you want to state ownership directly.

So both can be natural, depending on context.

A few parts are especially difficult for English speakers:

myggen

• The y has no exact English equivalent.
• A good trick is to say ee while rounding your lips as if you were saying oo.
• The ending -en is weak and unstressed.

sidder

• The vowel in the first syllable is short, a bit like i in sit.
• The dd is not a strong English d sound; Danish d is often softer.

på

• The vowel is roughly like aw in law for many learners.

arm

• The a is broad.
• In standard Danish, the r is not pronounced like a strong English r.

If you want, this whole sentence is worth learning by ear from native audio, because Danish pronunciation is often less straightforward than the spelling.