Learn Hardcore Russian: Мы увидели грузовик.

Мы = we; this is the subject.
увидели = saw; this is the verb, in the past tense.
грузовик = truck; this is the direct object, the thing that was seen.

So the structure is basically:

subject + verb + object
= We saw a truck

Why is the verb увидели and not видели?

Because увидеть is the perfective verb, while видеть is imperfective.

In this sentence, увидели presents the action as one completed event: we caught sight of a truck.

Very roughly:

мы увидели грузовик = we saw / caught sight of a truck as a complete event
мы видели грузовик = can mean we saw a truck too, but with a more imperfective viewpoint: the fact of seeing it, repeated seeing, or seeing without focusing on the completed moment

So увидели is very natural when you mean a single finished action.

How do we know увидели is past tense and plural?