One orthodox and self-evident theory is:
GDP = Money supply [x] Money velocity
If that velocity were constant (say a dollar gets spent 10 times a year) then the government could control the nominal GDP (counted in current-time inflating dollars) by simply controlling the money supply.
It's not constant, of course. The velocity changes, particularly when the system is under stress.
If the government doubled the money supply instantly, for instance, the real GDP would increase by (say) 5% immediately because people would activate productive capacity that was not there. It wouldn't double, however, because it takes time go to grow.
If velocity were constant prices would (almost) double right away. If the influx of money caused velocity to drop, however, the system could absorb extra money right away.
(Compare that to the theory of the Great Depression that when inequality puts all the dollars in the pockets of misers who save instead of spend, demand collapses.)
Another theory of inflation is that inflation happens if people expect inflation to happen.
If I think that prices are going to go up 5% next year, I am going to ask for a 5% raise, my employer will expect to raise prices 5% next year so it is ok...
People today do not believe in inflation so that governments can borrow and/or print a lot of money without provoking it.
That condition could go on for a long time until the psychology changes, in which case inflation returns.