553. Forgetting the Bracha Before Performing a Mitzvah
If a person is performing a mitzvah but he did not recite the bracha, then if the fulfillment of the mitzvah still ongoing, he may still recite the bracha even though he already performed his action. If the mitzvah is an act that has already been completed, he may no longer recite the bracha. Therefore, if a person put on tzitzis or tefillin, or if he sat in a succah, without reciting the bracha at the beginning, he may still do so while wearing the tzitzis or tefillin, or sitting in the succah. The same is true in all similar cases.
Conversely, if one slaughtered an animal without reciting the bracha, he may no longer recite the bracha after the act has been completed. Similarly, if one covered blood, separated terumah or tithes, or immersed in a mikvah without reciting the appropriate bracha beforehand, he may not recite the bracha afterwards. This is also true in all similar circumstances.