if there is a fine line between one thing and another, they are very similar although the second thing is bad.

There's a thin line between courage and foolishness. (often + between)

• You'd better believe it!
• you don't need this
• you have got to be kidding
• you haven't lived
• you haven't lived until
• you haven't lived until something
• You have to be cruel to be kind.
• You have to hand it to
• You have to hand it to someone
• you know
• You live and learn.
• You'll catch death!
• You'll catch death of cold!
• you'll catch it
• You'll catch your death!
• You'll catch your death of cold!
• You'll never hear the end of it.
• You mark my words.
• you may as well
• you may as well do something
• You may well ask!
• you might as well
• you might as well do something
• You must be joking!
• you name it