The word honesto exists in Spanish but the the best translation for honest is sincero, with ser.
- Dime una cosa y quiero que seas sincero. I want to ask you something. Please give me an honest answer.
- Me da much pena contigo pero tengo que ser sincero. Yo creo que estás comitiendo un error muy grave. This is awkward but I have to be honest with you. I think you’re making a big mistake.
- Quiero que seas sincera conmigo. I want you to tell me the truth.
As you can see from the last example, you can add con to specify with whom you are going to be honest. This can also be accomplished with an indirect object: Te soy sincero. I’ll be honest with you.
Notice that sincero is an everyday word in Spanish, whereas the English sincere is somewhat formal.
Para serte sincero
Para serte sincero/a is a stock expression that serves a prefacing function. You say it right before delivering the cold, hard truth.
- Pues para serte bien sincero, … Well, to be completely upfront with you, …
- Para serte sincero, yo fui el que … To be honest, I was the one who …
Note that para serte sounds like para hacerte. Don’t be fooled.
Con toda sinceridad
Con toda sinceridad = honestly, in an honest manner. Quiero que me respondas con toda sinceridad. I want you to answer honestly.