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.

             +--WN-+
             |     |
        The year when I lived in England was wonderful

      *The school when I lived in England was wonderful