Get Word Trainer in the App Store
This app helps you to learn the twos and threes in two modes. The main training mode works like flash cards, showing you a random two- or three-letter combination drawn from a fresh bag of tiles. Using the Yes and No buttons, you answer questions as to whether the play is allowed and whether it can be extended by adding an S. You can optionally see the definition of valid words immediately to help fix them in memory. The number of correct answers totals automatically as a percentage of right responses.
The second mode is to study the words in list format, with a standard iPhone index interface to jump to each letter in the alphabetized list. Since the word lists have definitions, they do not include S hooks that form different words. These extra S hooks are included in the training mode of the app, and the extra twos S hooks are certainly included in the threes word list. This part of the app also has an extra list of words containing a Q not followed immediately by a U, and can be supplemented in future updates by additional useful lists.
The default threes list does not contain the offensive words used in U.S. tournament play, but these can be included by modifying your settings for the app. For players outside of the U.S., you can set the app to use the more extensive list of words allowed in international tournament play, which by default includes the offensive words. Additional settings allow you to reset your scores once or begin with a clean score every time training starts.
Let me know what other features you think would be useful for future updates!