A lack of hamstring flexibility plays a key role in your yoga practice. (You’ll find a blog about hamstrings here.) Stiff hamstrings will limit you in a lot of yoga poses and can cause bad alignment in a lot of poses as well, because you are starting to compensate. This yoga class will give you more hamstring flexibility.
If you are like me and don’t have any flexibility by nature, go for slow classes. Yin yoga and vinyasa flow classes where you spent some time in a pose will help you improve your flexibility. Classes like Ashtanga are a nice work-out, but you won’t gain a lot of flexibility unless you’re flexible by nature. Easy and slow is the way to do it.