Ons moet die brief môre in die posbus sit.

Moet expresses obligation or necessity. In this sentence, it means something like must, have to, or need to.

So Ons moet ... means We must ... / We have to ...

In everyday English, have to is often the most natural translation, even though must is also correct.

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.

After a modal verb like moet, Afrikaans uses the main verb without te.

So you say:

• Ons moet die brief ... sit not
• Ons moet die brief ... te sit

Afrikaans does use te in some other infinitive constructions, but not after modal verbs.

Die means the.

It appears twice because there are two definite nouns:

• die brief = the letter
• die posbus = the mailbox / postbox

A helpful thing for learners is that Afrikaans uses die as the definite article for all genders and for singular and plural nouns as well. So unlike some other languages, the article does not change.

Not in the literal English sense of to sit down.

In Afrikaans, sit can also mean put or place, especially in everyday speech. In this sentence, in die posbus sit means put into the mailbox/postbox.

So here sit is being used in the sense of place or put, not be seated.

Afrikaans often uses in where English would distinguish between in and into.

So:

• in die posbus sit = put into the mailbox

The idea of movement into something is understood from the verb sit. Afrikaans does not always need a separate form equivalent to English into.

Posbus is a compound noun:

• pos = post / mail
• bus = box / container

So posbus literally means mail box or post box.

Depending on context, you could translate it as:

• mailbox
• postbox

Afrikaans makes a lot of compound nouns like this, so it is useful to get used to spotting the smaller parts inside longer words.

Afrikaans word order is somewhat flexible after the subject and finite verb, especially with things like object, time, and place.

In this sentence:

• Ons = subject
• moet = finite verb
• die brief = object
• môre = time expression
• in die posbus = place/direction
• sit = main verb at the end

This order is natural, but other orders are also possible, depending on emphasis. For example:

• Ons moet môre die brief in die posbus sit.
• Môre moet ons die brief in die posbus sit.

If môre is moved to the front, the verb still stays in second position: Môre moet ons ...

In the present tense, moet stays the same for all persons:

• ek moet
• jy moet
• hy/sy moet
• ons moet
• julle moet
• hulle moet

This is one of the easier parts of Afrikaans grammar: verbs usually change much less than in English or many other European languages.

Yes, ons can appear in more than one role in Afrikaans, but in this sentence it clearly means we because it is the subject before the verb:

• Ons moet ... = We must ...

So here there is no ambiguity.

Yes. You can say:

Môre moet ons die brief in die posbus sit.

That means the same basic thing, but it puts more emphasis on tomorrow.

This also shows an important Afrikaans rule: in a main clause, the finite verb usually stays in second position. So when Môre goes first, moet comes next, and ons moves after it.

The ô in môre shows a different vowel quality from a plain o. The word is roughly pronounced like MOH-ruh or MAW-ruh, depending on accent, though not exactly like any single English word.

The accent is part of the standard spelling and helps show pronunciation. It is not just decorative.