Y is used in certain idiomatic time and place expressions. It
connects quantity expressions to the head word of the expression
(i.e., the word that connects to the rest of the sentence):
             
             +------MVp-------+
             |          +--Y--+
             |          |     |
        We swam three miles away
	We swam three miles from the shore
	We swam three weeks ago
	We swam three hours after we saw you

Such expressions can usually be used as openers,
verb-modifying prepositional phrases, or noun-modifying
prepositional phrases: "We swam three weeks ago", "Three weeks
ago we swam", "The party three weeks ago was great". The word
carrying Y- is thus generally conjoined with (MVp- or CO+ or
Mp-).

Words that carry Y+ usually also carry {ND-}; this connects
back to a number expression. In some cases the ND- is optional
("We swam after we saw you"); in other cases it is obligatory
(*"We swam ago"). In some cases, the word carrying Y+ must
take a prepositional object or clause as well: "*We swam three
hours after"; such words therefore have "(J+ or Cs+) & Y+".
In other cases no connection may be made: "*We swam three
miles away the shore."

Yt is used in time expressions. Words like "ago" and "after"
have "Yt-"; "days", "weeks", etc, have Yt+, as well as
idiomatic expressions like "a_long_time". Yd is used in
distance expressions: "miles" and "feet" carry Yd+,
prepositions like "from" and "behind" carry Yd-.

Ya is used for a few adjectives that can take spatial
expressions as modifiers ("he is three feet tall"). Ye is used
in the expression "We swim every three weeks". Here "every" is
treated as the head; but in this case the number expression
follows the head instead of preceding it. 

Ys is used in time expressions like the following:

                         +------MVp-----+---Ys---+
     +-Wd-+--I--+-TO-+-I-+-Ox-+   +--ND-+    +NSa+
     |    |     |    |   |    |   |     |    |   |
   ///// I'd like.v to see.v her 100 times.i a day.p