Kommentar zu Bereschit 18:15

כי יראה וגו' כי צחקת FOR SHE WAS AFRAID… BUT THOU DIDST LAUGH — The first כי is used in the sense of “because”, giving a reason for the former statement—Sarah denied … because she was afraid; the second כי is used in the meaning of “but” — “and He said, ‘It is not as you say that you did not laugh, but thou didst laugh’.” For our Rabbis say (Rosh Hashanah 3a) that the word כי has four meanings: “if”, “perhaps”, “but”, “because”.

AND SARAH DENIED, SAYING. I wonder about the righteous prophetess:¹⁰³See above, Note 28, that Sarah was regarded as a prophetess. How did she deny that which G-d had said to the prophet,¹⁰⁴Abraham. In Megillah 14 a, Rashi quotes the Hilchoth Gedoloth listing the forty-eight prophets who arose in Israel, and the three patriarchs are listed among them. and also, why did she not believe in the words of G-d’s angels?
The answer appears to me to be that these angels who appeared as men came to Abraham, and he, in his wisdom, recognized them. They announced to him, “I will certainly return unto thee,¹⁰⁵Verse 10 here. and Sarah shall have a son.” And Sarah heard it, but she did not know that they were angels of the Supreme One, as was the case with the wife of Manoah.¹⁰⁶Judges 13:6. It is even possible that she did not see them at all. Therefore she laughed within herself in derision, just [as the word “laugh” is used in the verse]: He that sitteth in heaven laugheth, the Eternal hath them in derision.¹⁰⁷Psalms 2:4. For joyous laughter is [expressed in Hebrew as originating] in the mouth — Then was our mouth filled with laughter¹⁰⁸Ibid., 126:2. — but laughter originating in the heart is not spoken of as joyous. Now the Holy One, blessed be He, accused her before Abraham as to why the matter appeared to her to be impossible. It was fitting for her to believe, or she should have said, “Amen, G-d do so!” Now Abraham said to her, “Why did you laugh? Is anything too hard for the Eternal?” He did not explain to her that G-d had revealed her secret to him. And she, because of Abraham’s fear of G-d, denied it for she thought that Abraham had said so through recognition of the expressions on her face or because she had kept quiet and gave no expression of praise and thanksgiving or joy. And he said to her, Nay, but thou didst laugh. Then she understood that it was told to him in a prophecy, and so she remained quiet and did not answer a word.
It is proper that we also say that Abraham had not revealed to her what had originally been told to him: Indeed, Sarah, thy wife shall bear thee a son.¹⁰⁹Above, 17:19. Perhaps he waited until G-d would send her the announcement on the following day for he knew that the Eternal G-d will do nothing, but He revealed His counsel unto His servants the prophets.¹¹⁰Amos 3:7. It may be that due to his great diligence in fulfilling commandments, he was occupied with his circumcision and the circumcision of the many people in his house. Afterward, on account of his weakness, he sat at the doorway of the tent, and the angels came before he had told her anything.

כי צחקת. “but you did laugh.” On most occasions when we encounter the word כי after the word לא, it means “but.” Actually, the word כי can have four different meanings, depending on the context in which it appears (Rashi). The meanings are: if, maybe, but, for. Examples of כי meaning “but,” are Exodus 23,24 לא תשתחוה להם כי הרוס תהרסם, “do not bow down to them but destroy them utterly.” Or, Deut. 15,7-8: לא תקפוץ את ידך כי פתוח תפתח, “do not ball your hand into a fist, but open it wide, etc.”