Why there are only two cases …

Rule 1. Incident rays parallel to the principal axis after refraction through concave lens appear to diverge from principal focus F on the same side of the lens.

Rule 2. An incident ray passing through optical centre of a concave lens goes straight after passing through the concave lens.

Formation of images

1. Object at infinity. In this case, all the rays from infinity will come parallel and appear to diverge from F.
2. Object anywhere except at Infinity. Let the object AB be at point B. A ray of light from A parallel to the principal axis after refraction diverges towards DG. On producing back, it appears to diverge from the principal focus F. Another ray from A through C, goes undeviated.
   The two rays appear to meet at A'. Thus a virtual, erect, diminished image of AB is formed at A'B' i.e. between C and F.