What happens if you tear your ACL but don't have surgery to repair it and just try to carry on as normal?

Unstable knee. The ACL (anterior cruciate ligament) prevents the tibia (shin bone) from sliding forward from under the femur (thigh bone). This causes the knee to feel unstable and give way. It also increases the risk of damage to other structures, like the medial and lateral meniscus cartilage. The mensici act as shock absorbers and tearig them increases the risk of developing knee arthritis later in the future.
Depends. If you knee is unstable or participate in cutting and pivoting activities then you have an increased chance of having further injuries to the knee including cartilage and meniscus tears. Essentially, the knee can move around more than it should and this can harm the knee. If your knee is stable and you don't do intense sports then you can live without an acl a normal life.
Depends. If you do not engage in pivoting type athletic activities, you may be able to function with minimal limitations. Bracing and rehab may be all you need. Talk to your orthopaedic surgeon.