Add a letter to 'soa' that does not create a complete word.
In my haste, I forgot rule 2. When a letter is added it must be part of a real word, and words less than 3 letters do not count. Then the loser is the first player forced to add a letter that will complete a word.
EG. You are player five, and you are given 'stup', you can say 'i' or 'o' forcing player 6 say 'd' or 'r' both of which are losers. You could also say 'e' which would extend the word by at least two letters.