This is symbolic logic, based in: If X then Y or: X ) Y
So as an example:
X= I can only sing Y= If my mouth is open
X ) Y (the parentheses should be a backwards C than represents then)
In this example X represents singing and Y represents an open mouth.
If my mouth is open, it could mean that I might talk, eat or sing. My mouth being open doesn't imply that I'm singing, I could be drinking coffee or talking. So Y does not imply X.
But X implies Y
If I am singing my mouth must be open, as you can't sing with your mouth closed. You can hum, but you can't really sing without opening your mouth.
So: I can only sing if my mouth is open. X)Y or:
I can sing
If
my mouth is open