Ons was aan die saai.

Because Afrikaans verbs do not change the way English verbs do for different subjects.

The past form of wees (to be) is was for all persons:

• ek was = I was
• jy was = you were
• hy/sy was = he/she was
• ons was = we were
• hulle was = they were

So Ons was ... is completely normal Afrikaans.

This is a common Afrikaans way to show that an action was in progress.

So was aan die saai is like English:

• were sowing
• were busy sowing

It focuses on the action as ongoing, not just as a completed fact.

In this pattern, aan die + infinitive works as a fixed construction. Here, die is part of the progressive expression, not the normal article the in the usual sense.

So it is best to learn aan die saai as one grammatical pattern meaning in the process of sowing.

You will see the same structure in many other sentences, for example:

• Ek is aan die lees = I am reading
• Hulle is aan die werk = They are working

Here saai is the infinitive / base verb form, not the main finite verb of the sentence.

The finite verb is was.
That is the verb carrying the tense.

So in:

• Ons was aan die saai

the structure is roughly:

• Ons = subject
• was = finite verb, past tense
• aan die saai = ongoing-action expression

Also, even though saai can have other meanings in other contexts, here the grammar clearly shows it is being used as a verb.

Because after aan die, Afrikaans normally uses the bare infinitive, without te.

So you say:

• aan die saai
• aan die lees
• aan die werk

not:

• aan die te saai

You do get te in other constructions, such as:

• om te saai = to sow

So:

• Ons was aan die saai = We were sowing
• Ons was besig om te saai = We were busy sowing

Both are correct, but they use different grammar patterns.

Because in a normal Afrikaans main clause, the finite verb usually comes in the second position.

So the sentence order is:

• Ons = first element
• was = finite verb in second position
• aan die saai = rest of the predicate

This is a very important Afrikaans word-order rule.

You can compare it with a subordinate clause:

• Ek weet dat ons aan die saai was.

There, the verb order changes and was moves to the end.

The difference is mainly aspect:

• Ons het gesaai = we sowed / we have sown

  • this presents the action more as a fact or completed event

• Ons was aan die saai = we were sowing

  • this presents the action as ongoing at that time

So if you want to emphasize that the activity was in progress, aan die is the better choice.

Yes. That is a very natural alternative.

Both sentences can mean nearly the same thing:

• Ons was aan die saai
• Ons was besig om te saai

A rough difference is:

• aan die + verb often feels compact and very idiomatic
• besig om te + verb can feel a little more explicit, like busy doing

In everyday Afrikaans, both are common.

Mostly, you use it with action verbs that can really be seen as ongoing activities.

Good fits are verbs like:

• lees = read
• skryf = write
• eet = eat
• saai = sow

It is less natural with strongly stative verbs such as verbs meaning know, believe, or own, because those are not usually thought of as active ongoing processes.

So aan die is best learned as a progressive pattern for actions in progress.

Yes. It is complete if the context already makes that clear.

Afrikaans, like English, often leaves out information that the listener already knows. So Ons was aan die saai is fine on its own if everyone understands what activity is being discussed.

If needed, you can add more detail, for example a place, time, or object.

Just change the form of wees:

• Ons is aan die saai = We are sowing
• Ons was aan die saai = We were sowing
• Ons sal aan die saai wees = We will be sowing

So the part aan die saai stays the same, while is / was / sal ... wees shows the time.