Most people are aware that Hebrew verbs
come from three-letter roots. Each root is
conjugated in the 7 dimensions of
person, gender,plurality, tense, activity,
modality, and direct-object. For example
the root Shin Mem Resh means to watch.
The conjugations Shin-Mem-Resh-Tauv-Yud
and Nun-Shin-Mem-Resh-Nun-Vav mean
I watched and we were watched respectively.
The rules for Hebrew grammar are carefully described
in many modern books and are well known. Rashi will sometimes comment when a verse is using a rare conjugation
of an odd grammatical form.
When presenting grammatical Rashis my favorite
reference is the appendix in volume 5 of the Ibn Shoshan
dictionary. This very short appendix lists most
Verse Lv13-55a states
The underlined phrase corresponds to the translation of the Hebrew
word Hoo-Ka-BaYS which Rashi identifies as the passive infinitive
mode of the root Kuph-Beth-Samech. This conjugation is extremely rare. Modern
grammarians consider this to be an abnormal reflexive or passive causative.
Rashi's approach - the passive infinitive - is at least as defensible as the positions
of modern scholars. Furthermore Rashi's classification as passive infinitive is fully
consistent with all English translations. It is therefore the grammatical form that is most
consistent with the data.