Ok, you've been working on feet, then on posture, so the next thing is the hips. Most people are unaware of their hips, but tai chi requires you to move your hips to become effective.

The first thing is to practise the warm up exercises frequently. Tai chi warm ups are different from general sporting warm ups, in that they are designed to help you loosen up all your joints over time. After doing them for a couple of years, you will feel quite different in your body, because of this. Some exercises are aimed at specific joints and your warm ups will include exercises aimed at loosening the hips.

Don't worry if you don't feel loose to start with - it takes time and practise. Your instructor will show you how to use the warm ups to loosen each joint. They will also show you how to move your hip, when you are doing your tai chi form movements too. The great thing about these days is that you can use video to record yourself and see how you are doing, so you can check your hips (or whatever else) for yourself. When you're doing something, it might not be obvious to you why your instructor is trying to get you to alter your movement/position/whatever, but if you video yourself, you can start to see for yourself what you are doing well and what might need improving.

The hips are not the most obvious thing in the world to begin with - which is why your instructor will start by getting you to become aware of your feet, then next aware of your posture, before starting on your hips. However, once you start to feel your hips, your tai chi will start to look and feel as it should.