Learn Dutch: We laden de vrachtwagen in zodra hij aankomt. - We load the truck as soon as it arrives.

Zodra (as soon as) is a subordinating conjunction. This means it introduces a dependent clause, which forces all verbs to the very end of that clause. That is why aankomt is placed at the end.

Why is the separable verb inladen split, but aankomen is written as one word (aankomt)?

This sentence perfectly shows the difference between a main clause and a subordinate clause! In the main clause, the conjugated part of a separable verb goes in the second position, and its prefix goes to the end (We laden ... in). In the subordinate clause (after zodra), the verb is pushed to the end. When a separable verb is pushed to the end of a clause, its prefix and verb glue back together into one word (aankomt).

Why do we use hij (he) to refer to a truck (it)?

Nouns that use the article de are grammatically masculine or feminine. When pointing back to a de-word like de vrachtwagen, Dutch typically uses hij (he) rather than het (it), even for inanimate objects. You only use het to point back to het-words.

The sentence uses the present tense, but implies a future action. Is this normal in Dutch?