decir

"To say" or "to tell." Irregular in both the stem (e→i: digo, dices, dice) and the yo form specifically (digo, not the expected "deco"). The past participle dicho and gerund diciendo are also irregular and worth memorizing directly.

• ¿Qué dices? — What are you saying?
• Ella me dijo la verdad. — She told me the truth.
• ¿Cómo se dice "apple" en español? — How do you say "apple" in Spanish?

Confusing decir (to say/tell, reporting speech) with hablar (to speak/talk, the general activity) — "tell me" is dime, not *hablame*, which would mean "talk to me," a different request.

Often confused with hablar.

• expression "Dime con quién andas y te diré quién eres" — A centuries-old Spanish proverb — "tell me who you walk with and I'll tell you who you are" — appearing in 17th-century literature including Cervantes's Don Quijote and Quevedo's La vida del Buscón, still used to say a person's character shows in their company.
• expression "Del dicho al hecho hay mucho trecho" — A traditional Spanish proverb meaning there's a wide gap between saying something and actually doing it, recorded as far back as Juan de Mal Lara's Philosofía vulgar (1568) — built on dicho, decir's past participle.