Learn Swedish: Jag ändrade planen därför att han hade missat mötet. - I changed the plan because he had missed the meeting.

Just like in English, Swedish uses the past tense of the verb "to have", which is hade (had), followed by the supine form of the main verb. Here, that gives us hade missat to clearly show that he missed the meeting before you changed the plan.

Why does the sentence use missat instead of the simple past tense missade?

Whenever you use helping verbs like har (have) or hade (had), the following verb must be in its supine form, not the simple past tense. For regular verbs like missa (to miss), the simple past is missade, but the supine form drops the ending and adds a -t, resulting in missat.

Can I use för instead of därför att to mean "because" here?

Yes, you can! Both mean "because" and explain the reason for changing the plan. Därför att is slightly more formal and is often preferred in written Swedish, while för is shorter and extremely common in everyday speech.

How do the endings on planen and mötet remind us if they are en or ett words?