Try reading Kant's "Prolegomena" -here's a quote from [282-283] pg 30 'Library of Liberal Arts' edition. In the section 'First Part of the Main Transcendental Problem ... How is Pure Mathematics Possible?'
Now, the intuitions which pure mathematics lays at the foundation of all its cognitions and judgments which appear at once apodictic and necessary are space and time. For mathematics must first present all its concepts in intuition, and pure mathematics is pure intuition; that is, it must construct them. It if proceeded in any other way, it would be impossible to take a single step; for mathematics proceeds, not analytically by dissection of concepts, but synthetically, and if pure intuition be wanting there is nothing in which the matter for synthetical judgments a priori can be given. Geometry is based upon the pure intuition of space. Arithmetic achieves its concept of number by the successive addition of units in time, and pure mechanics cannot attain its concepts of motion without employing the representation of time. Both representations, however, are only intuitions; for if we omit from the emirical intuitions of bodies and their alterations (motion) everything empirical, that is, belonging to sensation, space and time still remain, which are therefore pure intuitions that lie a priori at the basis of the empirical. Hence they can never be omitted; but at the same time, by their being pure intuitions a priori, they prove that they are mere forms of our sensibility, which must precede all empirical intuition, that is, perception of actual objects, and conformably to which objects can be known a piriori, but only as they appear to us."
What Kant is saying is no more than this: Space and time are pure, universal concepts of the understanding and without them there is no possibility of anything (no sense experience ... no empirical sense of any kind ... no grounds for anything that we know or experience). They must precede everything else for everything else to exist. Consequently, they must be a priori and they do in fact form the a priori grounds for our possible experience of reality.
Whether or not you agree with Kant is another matter. Personally, I do happen to agree with him. And he's not all that difficult, heck ... after reading him for the last 30 damn years.
Hope this helps.