Questions & Answers about Ons moet die brief môre in die posbus sit.
This is a very common Afrikaans pattern.
When you use a modal verb like moet, kan, wil, or sal, the modal verb is the finite verb and comes early in the clause, while the main verb goes to the end in its basic form.
So:
- Ons moet = We must
- ... sit = put
That is why you get:
Ons moet die brief môre in die posbus sit.
This is similar to Dutch and German-style word order.