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
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
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.