Learn Italian: Chi ci tiene all'onore dice sempre la verità. - Whoever cares about honor always tells the truth.

The verb tenerci is a fixed pronominal verb where ci is baked into the meaning to change tenere (to hold) into tenerci (to care about). Even when you explicitly state what you care about using the preposition a (like all'onore), you must keep the ci.

Why are the verbs tiene and dice in the singular if "whoever" can refer to multiple people?

The relative pronoun chi (meaning "whoever" or "those who") is always treated as grammatically singular in Italian. Any verbs attached to it, like tiene and dice, must be in the third-person singular, even if you are talking about a whole group of people.

Why does the sentence use all' (a + l') before onore instead of a preposition like di or su for "about"?

The verb tenerci specifically requires the preposition a to introduce the thing you care about. It helps to think of the literal meaning as "to attach importance TO something." Here, a combines with the definite article l' to form all'onore.

Could we use a verb like importare (to matter / to care) for this sentence instead?