How hard should I try when I practice yoga poses?

You should give just 80% physical effort, but 100% mental effort.
Yoga is a practice of observation. If you use 100% physical effort, your mind will be overloaded with the physical stress. It will have no energy left to observe and learn about the body. You just need to do enough physically to provide a signal for the mind to notice where the issues are in your body. When your mind notices, it will naturally trigger biological changes in the body to resolve the issues.

Just because you are not pushing 100% physically, does not mean you will not improve. Through observing and resolving issues in your body, it becomes more efficient. The intensity of practice that took your 80% effort will start to feel like 60% effort. Then you can increase the practice intensity to come to a new 80% effort. Now you are still practicing at 80%, but working a higher level. You can keep improving your body this way without causing excessive strain.

​The difficulty is that when you are not trying 100% physically and you have more mental energy left, it is easy for your mind to drift. So the real challenge then is to maintain 100% mental effort on observing your body - to discover, and develop deeper connections with it.