*** Guide-to-Links ***
WN is used to connect "when" phrases back to time-nouns like "year"
and "day". In such cases, WN is acting like a noun-modifier;
such modifiers would normally use M-. However, "when" cannot
normally be used in this way, as the second sentence shows;
only with time nouns. Thus time-nouns have an option WN+
(along with an optional @M+); "when" has WN- conjoined with
Cs+, connecting to a subordinate clause.
The year when I lived in England was wonderful
*The school when I lived in England was wonderful
Grammar Documentation Page.