Learn Spanish (Spain): Le digo la verdad.

Lo is a direct object pronoun used for the exact thing being acted upon. Le is an indirect object pronoun used for the person receiving that thing. In this sentence, the direct object is la verdad (the truth) because that is what you are telling. You are telling the truth to him, making him the indirect receiver, so we use le.

The translation says "I tell him", but couldn't le also mean "her"?

Yes! The pronoun le is gender-neutral. It can mean to him, to her, or even to you (formal, usted). If it isn't clear from the context of the conversation who le refers to, Spanish speakers often add a él, a ella, or a usted to the sentence for clarification (for example: Le digo la verdad a ella).

Why does le come before the verb digo?

In Spanish, object pronouns like le must be placed immediately before a conjugated verb. Unlike English where we say "I tell him", Spanish puts the receiver first, so the structure works like "To him I tell".

Where is the word for "I" in this sentence?

It is built right into the verb! Digo is the "I" form of the irregular verb decir (to say or to tell). Because the word digo already makes it clear that "I" am doing the action, you don't need to include the subject pronoun yo unless you want to add special emphasis.