All imperatives are expressed by an 'ought'. By this they mark the relation of an objective law of reason to a will that is not necessarily determined by this law in virtue of its subjective constitution. A hypothetical imperative is not simply a conditional 'ought' . Hypothetical imperatives merely indicate an action as what one must do, consistently with pursing a given end. You could decide not to go to Flint.
Categorical imperatives say what, under certain circumstances, one ought to do.
Thus, unlike a merely hypothetical imperative, one can conclude from them that, if the circumstances obtain, one really ought so to act. Part of what Kant means in the Preface by the 'absolute necessity' he says is an essential component of our ordinary idea of moral duty or obligation, is that, as we ordinarily think of them, moral 'ought''s are categorical imperatives. That is what the little argument that we looked at last time was supposed to answer. The universal law formulation. Kant means a principle that includes the agent's motive or reason for acting.