What are some good exercises to stabilize my knee after an acl tear?

Hamstring exercises: Both the ACL and hamstings prevent excessive abnormal forward translation of the knee, but the ACL is a primary stabilizer and the hamstrings are secondary restraints. After the ACL is torn, the hamstings, by default, become more important in limiting abnormal forward translation and instability of the knee. Strengthening them minimizes this unwanted motion and instability.

PT: Your knee has both static stabilizers (acl and pcl) and dynamic stabilizers (quads and hamstrings). Hamstring and quadricep strengthening is usually incorporated into acl rehab. Leg extension and flexion exercises in a brace can be helpful. Talk to your physical therapist.