Inevitably means “this is certain to happen, and you cannot avoid it.” You use it when the result is locked in, even if people hope otherwise. It often sits at the front: “Inevitably, they started to argue.” It can also sit before the verb: “If you leave milk out, it will inevitably go sour.” It is a little more formal than “of course” or “sooner or later,” and it is stronger than those — it is not only likely; it is bound to happen. Though actually, people do use it a bit loosely, the way they use “always,” for things that happen so often they feel unavoidable, even when a careful person could still imagine an exception.