Or HaChaim on Leviticus 15:28/אור החיים על ויקרא ט״ו:כ״ח

1 ואם טהרה מזובה וספרה, But if she be cleansed from her issue, then she shall number, etc. We have to try to understand the use of the past tense for the word וספרה instead of the future tense, i.e. ותספור. Precisely when is the count to commence? If the count is to commence immediately, why does the Torah not write ותספור? If it is to occur only after she has become purified, the words "afterwards she shall be clean" do not make sense. We may have to explain this in light of what Torat Kohanim wrote on the meaning of the words טהרה מזובה. They interpret it as a cessation of the issue. The Torah tells us how we are to know that she may be considered cleansed from her issue, i.e. from the time the flow of blood stops. Her "purity" is then sufficient to permit her to begin the count of seven days (during which the flow must not recur). Rashi concurs with this interpretation in Megillah 8. He writes that she may begin counting without first having to immerse herself in a ritual bath.

2There is another way of interpreting our verse. The words ואם טהרה, "if she is cleansed," mean that the flow of blood has stopped; after all, this is the plain meaning of "she is clean from her flow of blood." You should not think, however, that the impurity has departed from her; the only thing which has departed from her is the contaminating issue. This is why the Torah underlined this by the extraneous word מזובה, "from her issue." What has not departed is the impurity transmitted to people touching the afflicted person. She is still ritually impure and has to count seven days during which she must be free of renewed symptoms. This is why the wording וספרה, a past tense converted by the use of the וו ההיפוך, the letter ו which reverses the tenses is in order.