Learn Danish: Albuen gør ondt når jeg løfter armen. - My elbow hurts when I lift my arm.

In Danish, when it is obvious whose body part you are talking about, you usually use the definite form (the elbow/arm) rather than a possessive pronoun like min (my). Using min albue here wouldn't be grammatically wrong, but albuen sounds much more natural to a native speaker.

Why is ondt in the neuter t-form even though albuen is a common-gender (en) word?

The phrase gøre ondt (to hurt or cause pain) is a fixed expression. Literally, it means "to do bad" or "to do evil." Because ondt functions as a fixed part of the verbal phrase here rather than an adjective describing the elbow, it stays in its neuter form regardless of the noun it relates to.

The sentence uses når for "when". Could I use da instead?

Not in this case. In Danish, når is used for present and future events, as well as repeated events in the past (meaning "whenever"). Da is strictly used for a single, specific event in the past. Since this sentence is in the present tense (gør, løfter), you must use når.

Why is the word order normal (jeg løfter) after når, instead of swapping the verb and subject?

Når starts a subordinate (dependent) clause. In Danish subordinate clauses, the subject always comes immediately after the conjunction, followed by the verb. Inversion (putting the verb before the subject) only happens in main clauses when something other than the subject comes first.