only if
Only in the event that something happens (will something else happen or be the case). A: "Can I go to Janet's party this weekend, Mom?" B: "Only if you finish all your homework and your chores." Only if three senators cross the aisle in favor of the bill will it be able to pass through the Senate.
ˈonly if
used to state the only situation in which something can happen: Only if a teacher has given permission is a student allowed to leave the room.Only if the red light comes on is there any danger to employees.