I cannot know for certain if it was carried out.
The fact is, we can't know for certain.
You can never know for certain if someone is really interested in you.
I'd have to check the dendrochronology first - to know for certain.
I would have to know for certain that she felt the same way.
I do not know for certain what she is going to do.
No estoy seguro de lo que va a hacer.
Because I know for certain what has happened to the human race.
Porque sé con certeza. qué le ha pasado a la raza humana.
You never know for certain what will work unless you try it.
Nunca se sabe con certeza qué funcionará a menos que lo intente.
You will never know for certain until you are tested.
Nunca lo sabrás con certeza hasta que seas puesto a prueba.
We don't know for certain that it's a kidney infection.
No sabemos con certeza que se trate de una infección renal.
I won't return soon, this I know for certain.
No voy a volver pronto, lo sé a ciencia cierta.
But what I know for certain is that she needs help.
Pero lo que sé con certeza es que ella necesita ayuda.
And we don't know for certain that's what you have.
Y no sabemos con certeza si eso es lo que tienes.