since

• 既然如此

  if this is the case

• 你既然不知道，那就不要说。

  Since you don't know, don't talk about it.

• 既然你同意，那我也不反对。

  Since you agree, I will not oppose it either.

• 既然他不去，那我去。

  If he's not going, I'll go.