How did Kant combine transcendental idealism with empirical realism?

Immanuel Kant combined transcendental idealism with empirical realism by distinguishing between how we experience objects and the nature of those objects themselves.

Kant argued that space and time are not properties of "things-in-themselves" but are a priori forms of human sensibility. Because being in space and time is a necessary condition for any object to be experienced, the reality of objects external to the mind is guaranteed within the realm of experience. This constitutes his empirical realism: the view that objects external to the mind truly exist as they appear to us.1

Simultaneously, his transcendental idealism maintains that what we perceive are only "appearances" constructed by the mind using categories. By asserting that objects must conform to the mind’s structures to be known, Kant avoided the need to infer the existence of external objects from hidden causes. Instead, he established that while we can have valid knowledge of objects of possible experience, the "things-in-themselves" remain forever beyond the reach of human knowledge.213