Formal:
* Être sûr: This is the most direct translation, meaning "to be sure." You can use it in formal settings like writing or speaking to someone in a position of authority. Example: "Je suis sûr que vous trouverez la solution." (I am sure you will find the solution.)
Informal:
* Être certain: This means "to be certain" and is slightly less formal than "être sûr." Example: "Je suis certain qu'il est arrivé." (I am certain he has arrived.)
* Être persuadé: This means "to be persuaded" and implies a stronger level of conviction. Example: "Je suis persuadé que c'est la bonne décision." (I am persuaded that this is the right decision.)
Other options:
* En être sûr: This literally translates to "to be sure of it" and is another common way to express certainty. Example: "J'en suis sûr!" (I am sure!)
* Ne pas en douter: This means "not to doubt it" and is a more emphatic way to express certainty. Example: "Je ne doute pas que vous réussissiez." (I have no doubt you will succeed.)
The best way to choose the right phrase depends on the specific context and the level of formality you want to achieve.