Learn Icelandic: Stundum meiðist hann.

The base verb is meiða, which means "to hurt" or "to injure" someone else. When we add the -st ending to make it meiðast (the middle voice), the action turns back on the subject. So here, it means "he gets hurt" or "he hurts himself".

How is the form meiðist built from the regular present tense?

The regular "he" form of the base verb is meiðir (he hurts). To make the middle voice, Icelandic drops the final -r and adds -st. That is why meiðir becomes meiðist.

Why is the word order meiðist hann instead of hann meiðist?

This happens because of the verb-second (V2) rule. In Icelandic, the main verb generally has to be the second element in a sentence. Because we started the sentence with the adverb Stundum (sometimes), the verb meiðist must come second, pushing the subject hann to the third spot.

Do we use meiðist for emotional hurt, too?

No, meiðast is used specifically for physical injuries, like twisting an ankle or getting a bruise. If someone's feelings get hurt, Icelandic uses a different verb, such as særast.