Space is the form of all appearances of outer sense, the subjective condition of sensibility, and as such can be given a priori in the mind

The passage

Space is nothing other than the form of all appearances of outer sense, that is, the subjective condition of sensibility under which alone outer intuition is possible for us; and since the subject's receptivity to be affected by objects necessarily precedes all intuitions of these objects, it can be understood how this form of all appearances can be given in the mind before all actual perceptions, hence a priori.

Conclusion (b) from the exposition of space, A 26 / B 42, first paragraph (AA 3:55.09–18). The first sentence is the statement's first half (3:55.09–11). The second half keeps the sentence's modal form: Kant says that it »läßt sich verstehen, wie« the form of all appearances »a priori im Gemüthe gegeben sein könne« (3:55.13–15), an explanation of how it can be so given, resting on the receptivity of the subject preceding all intuitions of objects (3:55.11–13); the claim does not turn that into a flat assertion that the form is given a priori, which the metaphysical exposition asserts separately (c-kant-space-a-priori, A 24 / B 38). What is subjective here is a condition of sensibility, not, in Kant's words, of the intellect; Schopenhauer's Anhang sentence that shares the premise says »Form unsers Intellekts« (c-schopenhauer-causality-sensation-and-space-are-subjective), and the comparison's shared premise is worded on the part both assert, that space is a subjective form. Space as no determination of things in themselves is conclusion (a), c-kant-space-not-of-things-in-themselves.