Almost all of my classes start with working on standing in tadasana for a few minutes. All of the poses that follow are little more than a reflection of tadasana in different shapes.
Much of the magic or misery that occurs as a result of our standing posture happens in the area known as the groin, where the leg meets the pelvis. There should be a crease in the hips, or depth in the groins when standing in tadasana as well as every other pose.
Warrior two is no exception but it tends to fall apart in the back leg for so many students.
Warrior two is a pose that is much easier for someone with open hips. Correct alignment means that the front knee tracks directly on top of the front ankle and the back outer hip (greater trochanter) lines up with the ankle bone.
If the hips are tight it is very hard if not impossible to achieve both of these objectives. With tight hips if the front knee aligns correctly the back hip will likely be pushed forward and vice versa. If the back hip aligns well, the front knee probably collapses to the inside a bit.
If the hips are tight, the only way to achieve the proper alignment of both legs is to allow the upper body to pitch or curve forward. This usually increases the arch of the lumbar spine and moves the spine far away from the verticality that it seeks above the pelvis.
All poses are technically difficult and muscle and postural imbalances often make finding the ideal unimaginable. But knowing the objective and be patient with yourself on the journey towards the goal is the true point of the practice.