554. Is There a Bracha on a Preliminary Mitzvah?
The only mitzvah where the bracha is recited after its performance is the immersion of a convert. This is because he can’t say “Who sanctified us with His commandments and commanded us…” until he has immersed, as this does not apply to him. This exception was instituted because the convert was inherently unable to recite the bracha before performing the mitzvah.
When performing a mitzvah completes one’s obligation, the bracha is recited before performing it. When another mitzvah follows that mitzvah, the bracha is not recited until the other mitzvah is performed. Therefore, when one makes a succah, a lulav, a shofar, tzitzis, tefillin or a mezuzah, he does not recite a bracha that God “commanded us to build a succah” or “…to write tefillin,” etc. since another mitzvah follows. Rather, the bracha is recited upon sitting in the succah, shaking the lulav, hearing the shofar, wearing the tzitzis or tefillin, or affixing the mezuzah. Conversely, one does recite the bracha before installing a guardrail on his roof and in all similar cases where no other mitzvah follows.