Verse Lv06-02b states Usually a fountain or a pit-a gathering of water, shall be ritually pure; but that which touches their carcass shall be ritually impure. Rashi, commenting on the word usually expounds: Fountains and pits usually confer ritual purity - when they are part of the ground. [But a movable water tank even if it had the prerequisite size would not confer ritual purity.]

Advanced Rashi: The official Talmudic rule used by Rashi is that The Hebrew Aleph Caph connotes limitation and hence Rashi limits the verse's applicability to ground pits and fountains.

In my article The Meaning of Ach which may be found on the world wide web at http://www.Rashiyomi.com/ach.pdf I introduce a method to make this Talmudic principle more intuitive: I suggest that the Talmudic principle can be mirrored by a punchy English translation whose nuances spontaneously capture the Talmudic logic. In this case I suggest that the word usually captures the Talmudic nuance of limitation. The translation of Aleph Caph as meaning usually explains many Talmudic interpretations and is a natural way of capturing the idea of required limitation in the verse.