De bestelling was compleet maar kwam te laat aan.

Because bestelling is a de-word in Dutch. Nouns in Dutch have to be learned with their article, so it is:

• de bestelling = the order

There is no simple rule that lets you always predict de or het, so it is best to learn bestelling together with de.

Bestelling usually means order in the sense of something you ordered:

• an online order
• a food delivery order
• a purchase order

So in this sentence, de bestelling is the package, meal, or set of items that was ordered.

Was is the past tense of zijn (to be).

Here, it links the subject to the adjective compleet:

• De bestelling was compleet = The order was complete

So was does not mean became or existed here; it simply means was in the normal English sense.

Dutch often leaves out the subject in the second part of a sentence when it is the same subject as in the first part.

So:

• De bestelling was compleet maar kwam te laat aan.

means the same as the fuller version:

• De bestelling was compleet maar de bestelling kwam te laat aan.

But repeating de bestelling sounds unnecessary and unnatural.

This is very common in Dutch, especially with conjunctions like en and maar.

Because the verb is aankomen, which is a separable verb.

Its basic form is:

• aankomen = to arrive

In a main clause, the verb splits:

• the conjugated part goes near the front: kwam
• the particle goes to the end: aan

So:

• kwam ... aan = arrived

This is a very important Dutch pattern. For example:

• Ik kom aan. = I arrive / I am arriving.
• Hij kwam laat aan. = He arrived late.

The past tense is formed from the base verb komen, not by changing the whole separable form as one piece.

• infinitive: aankomen
• base verb: komen
• past tense of komen: kwam
• separable particle: aan

So:

• aankomen → kwam ... aan

Also, komen is a strong verb, so its past tense changes vowel:

• komen → kwam
• plural: kwamen

Here, compleet means complete in the sense that nothing was missing.

So it suggests something like:

• all items were included
• the order was not incomplete
• everything that should have been there was there

A close alternative in Dutch could be volledig, though compleet is very common in everyday speech.

This is an important difference:

• laat = late
• te laat = too late

So:

• kwam laat aan = arrived late
• kwam te laat aan = arrived too late

The word te often means too in the sense of more than is acceptable or desirable.

Because with a separable verb in a main clause, the particle usually goes to the end.

Here the verb is aankomen:

• kwam goes in the normal finite-verb position
• aan moves to the end

Anything else, such as te laat, comes before that final particle:

• kwam te laat aan

This end position is normal for separable verb particles in Dutch main clauses.

Yes. Dutch can also use arriveren, but aankomen is much more common and natural in everyday language.

Compare:

• De bestelling kwam te laat aan. = natural, everyday Dutch
• De bestelling arriveerde te laat. = understandable, but a bit more formal or less common in many everyday situations

So for normal spoken and written Dutch, aankomen is the better verb to learn first.