Torah Study 24/7 Yud Gimmel Tammuz, 5777
The berachah on certain mitzvos is said using the word על, while on others we use a למ"ד. One of the rules is that if the berachah is being said after the mitzvah was performed, we say על.
This is why the berachah for Torah study is על דברי תורה. When someone is about to study, the obligation to study precedes him, because we are commanded to learn every day and moment. For this reason, we use the word על.