Instability. Tears of either can make your knee feel unstable, but in different planes. The MCL tear will give you more pain over the inner side of your knee.
Yes. Both are stabilizing ligaments in the knee. The ACL (anterior criciate ligament) prevents the tibia (shin bone) from sliding forward from under the femur (thigh bone). The MCL (medial collateral ligament) prevents the knee from going into a medial or valgus ("knock-knee') postion. They can occur alone or in combination with each other.
MCL will heal. Mcl tears hurt on the inside of the knee when a stress is applied to the inside of the foot. Acl tears cause instability in full extension and hurt when the knee shifts. Mcl tears usually heal in 2-3 months. Acl tears rarely heal.
Sometime not much. It sometimes can be difficult to distiguish the two from symptoms patients experience alone. That is why a good physical exam is key to making the diagnosis.
Effusion (swelling) An acl tear can often include an mcl tear that occurs at the same time. An acl tear generally causes bleeding in the knee joint caused by tearing the blood vessels around the acl. This causes a very fast swelling of the knee joint (usually without the hour). An isolated mcl tear usually does not swell inside the knee and causes pain on the inner portion of the knee, where the mcl tore.