It is my understanding the the tactics trainer is only intended to grill you with random themes. You are probably looking for Chess Mentor training, which lets you do precisely what you asked for.
how to practice on Tactics Trainer?
Sort:
I'm a bit embarassed to ask this, but how does one practice on Tactics Trainer in such a way as to work ONLY on one aspect/theme/category at a time? I've tried to practice checkmates by selecting "checkmates" as the category and "common checkmating patterns" as the theme, but after one problem it goes straight back into some kind of random selection of problems. Do I have to keep re-selecting the theme and category over and over again?