Multiple basic techniques & combinations
Beginners often find combinations of techniques very difficult to perform, especially because in Taekwondo you have to use both sides of your body. The simplest of blocks and punches can become complicated if they are shown in a slightly different way. The only advice I can give is to think about one technique at a time and don’t get carried away with the technique as a whole.
When grading it is quite common to get combinations that you have never practiced in that order before. Concentrate and do the techniques in order, one move at a time, thinking about getting from A to B. The more you practice the better your coordination and combination basic training will become.