When using the grammar method Rashi will instruct students in Hebrew grammar similar to the instruction found in modern day textbooks.

A classical part of all Hebrew grammar courses are the rules for conjugation of verbs. All verb roots may be conjugated in 7 dimensions: a) passivity (active-passive) b) modality(will-should), c) person (I, you, he), d) plurality, e) tense (past-future-infinitive), f) gender, g) object, h) connective preposition. A standard quick good summary for conjugation is the appendix of the Ibn Shoshan dictionary.

Hebrew roots are considered three-letter objects. The midieval nomenclature for these three letters are Pay, Ayin, Lamed while the current notation is 1,2,3. For example a Biblical root whose first letter is Yud would be called in midieval terminology a Pay Yud root while in modern terminology it would be called Yud-2-3 root. Both terminologies indicate that the first letter of the root is Yud. The conjugation rules for Yud-2-3 roots differ from the conjugation rules for ordinary roots. Table 2 of Ibn Shoshan gives the conjugation rules of these roots over the four dimensions of activity, tense, gender, person.

Young Yeshiva students are frequently taught (or forced to memorize) these tables. Studying these tables immediately yields the Rashi comment: The Yud changes to a Vav in the passive, causative, and passive-causative (Nifal, hifil, hofal) modes. The Yud however remains in the Qal, Piel, Pual, and Hithpael modes.

Advanced Rashi: The above is a paraphrase of Rashi. Rashi actually only literally says that: The Yud changes to a Vav when the mode is passive. But as shown above this is only partially true. For example the Yud does not change to a Vav in the Pual - Passive-intense mode. Similarly the Yud changes to a Vav in the active-causative mode.

We conclude that the Rashi comment is only a rough approximation and not 100% accurate. I conjecture that Rashi used such rough approximations as a pedagogic technique. Rashi taught the basic rule which experienced teachers would then amplify on.

The above analysis is very useful to those who think that the best approach to Rashi is to be literal and defend every minutae of Rashi. Such an approach does not always yield true results. To recap: Rashi said that Yud changes to Vav in passive situations but as seen this is only true in two out of the three passive tenses. Rashi expected the teacher and student to fill in the missing details.

Such an approach to Rashi is mature, deep and enriching. It sees Rashi as blazing a path while leaving it to others to smooth the path and pave it so it is smooth.