We can get into the house only if you have a key. CAN Only if ... we get into the house.
Only if YOU HAVE A KEY CAN we get into the house.
It the weather gets any colder, we'll turn on the heating. GET Should ... we’ll turn on the heating.
Should THE WEATHER GET ANY COLDER we’ll turn on the heating.
He had just entered when the telephone rang. SOONER No ... the telephone rang.
No SOONER HAD HE ENTERED THAN the telephone rang.