How do you say I have always known this in French?

There are several ways to say "I have always known this" in French, depending on the nuance you want to convey:

Formal:

* Je le sais depuis toujours. (This is the most literal translation and is the most formal option.)

* J'ai toujours su cela.

* J'ai toujours été au courant de cela. (This emphasizes that you were aware of it)

Informal:

* Je le sais depuis toujours. (This works for both formal and informal situations.)

* Je savais ça déjà. (This is more casual and implies you knew it before the other person.)

* Je le savais bien. (This implies you were certain of it all along.)

With emphasis:

* J'ai toujours su que c'était comme ça. (This emphasizes that you have known it for a long time.)

* Je le savais depuis le début. (This emphasizes that you knew it from the beginning.)

The best translation will depend on the specific context and your desired tone.