Learn Italian: Non spiegarglielo! Deve usare il righello prima. - Do not explain it to him! He has to use the ruler first.

For the negative informal imperative (telling someone not to do something), you use non plus the infinitive verb, which here is spiegare. To attach the double pronoun glielo, you drop the final -e of the infinitive and stick it on the end. You could also place it before the verb, like Non glielo spiegare! Both ways are completely correct.

How did gli (to him) and lo (it) turn into glielo?

When the indirect pronoun gli combines with direct object pronouns like lo, la, li, or le, they merge into a single word connected by an extra -e-. Therefore, gli plus lo becomes glielo.

The verb deve can mean he, she, or it has to. How do we know it means he has to here?

We get the context from the first sentence! The pronoun gli inside glielo specifically means to him. Because we know we are talking about a male from that pronoun, it carries over as the implied subject of deve in the second sentence.

Could the word prima go somewhere else in the second sentence?