Translation:Shulchan Aruch/Orach Chaim/434

1. After checking, one should be careful with the chametz that he intends to keep, so that he does not need another check. For example, if a mouse passes by it or if some of the chametz is missing, such as if he placed ten loaves of bread and found only nine, he should ensure that it is properly covered. However, if he simply does not know whether any of the chametz is missing or not, and if he forced a utensil onto it and did not find any chametz, he does not need to check again. Certainly, if a person has taken care of it, he will force a utensil onto it, or hang it in the air, or put it in a box in a place where mice cannot reach it.

2. After the examination, immediately at nightfall, we will nullify it and say: Any leaven that is in my possession, which I have not seen and have not removed, shall be nullified and become like the dust of the earth. Rema: And one should declare the nullification in a language that is understood [as stated by Maharil, son of Rabbeinu Nosson], and if one said it in Hebrew: 'Any leaven includes both leaven and sourdough' [Tur, Orach Chayim, Chapter 434]. However, in other languages, one must mention each one separately [Darchei Moshe]. And it is good to go back and nullify it again another time on the fourteenth day, at the end of the fifth hour, before the sixth hour arrives, as once the sixth hour arrives, it becomes forbidden, and there is no opportunity to nullify it. Rema: And one should not nullify it during the day, but rather after burning the leaven, in order to fulfill the commandment of burning one’s leaven. (Mahariv)

3. In the nullification during the day, one should say 'whether I have seen it or not, whether I have removed it or not'.

4. His messenger can nullify it (and when the messenger nullifies, he should say 'the leaven of so-and-so shall be nullified, etc.') [Responsa of Maharil]. If the man is not at home, he should nullify it in his place, and if he does not do so, it is better that his wife nullifies it.