How many times have you heard that?
This is a common issue amongst all levels of practitioners and not just beginners. And the teacher isn't pointing this out to stunt one's progress in class (unless they are not a real teacher), but they are doing so because:
1. They care that you learn the yoga properly.
2. They care about your body.
3. They care about their teaching methods.
4. They are all in all a caring teacher.
Keeping your form is a very crucial part of your practise, and in order to learn the yoga you are practising properly you must first be able to keep your alignment, and you must also open the opportunity within your mind, for your body to learn the posture properly.
Once you get comfortable with a posture and you have the correct form and alignment then you should be pushing as much as possible to get as much depth as possible and your body will be ready, like a flower petal blooming to open, but until then be patient and be aware of what you are doing.
It is totally understandable that you would like to do a posture to it's optimum and aim to look to best in class, but yoga is not about looking the best, nor is it about incorrect posture. Posture is one of the most important practices in yoga, and without correct posture, the body will only pick up unnecessary bad habits.
If you are struggling with this always reach out to the teacher as they will be able to help and guide you in the right direction.
Thanks for reading.