Learn Swedish: Min axel är så stel att armen värker. - My shoulder is so stiff that my arm aches.

It connects a cause with its result, just like "so... that" in English. The word så (so) modifies the adjective stel (stiff), and att (that) acts as a bridge to introduce what happens as a consequence: armen värker (the arm aches).

The English translation says "my arm", but the Swedish sentence says armen (the arm). Why?

In Swedish, you generally use the definite form of a body part (like armen instead of min arm) when it is already obvious whose body it belongs to. Since you established it is your shoulder (min axel) in the first clause, the listener knows it is also your arm that hurts!

Why is the adjective stel instead of stelt or stela?

Adjectives in Swedish must match the noun they describe. axel (shoulder) is a singular en-word, so we use the base form stel. If it had been an ett-word like ett knä (a knee), it would be så stelt, and if it were plural (axlar), it would be så stela.

Can I use gör ont instead of värker here?