Sentence Surgery
Lift the extra phrase out of the sentence, then fit the right verb into the gap.
This engine’s own main skill is Subject-Verb Agreement. It practises complete a finite clause alongside it.
The subject of this sentence is "box", not "pencils". "Of coloured pencils" is a prepositional phrase that only describes the box, and a subject never lives inside one. One box is singular, so it takes the singular verb "was". Lift the phrase out and the sentence tells you the answer itself.
What to say if they get stuck
- Grab the little handle on "of coloured pencils" and drag the whole phrase upwards.
- Read what is left behind: "The box ___ on the art room shelf." What is the sentence actually about?
- The subject can never hide inside a phrase beginning with "of". One box, so: "was".