這個問題要簡單解決的話,就睇子句中是【吞咗noun】?還是【吞咗prep+n】(即adverbial)/【吞咗adverb】。
which 是relative pronoun 即是有能力【吞n】而已;
when 是relative adverb 即是有能力【吞adverbial】(prep+n)。
We met last summer ________the temperature was blazing hot.
子句加返要形容的【主句部份】進去,會是:the temperature was blazing hot【 last summer】。而last summer這裡是adverb組合(不單吞summer一字,還吞埋last), 所以應該用when 而不用which.
如例子是:
We always have a summer ________ blazes people.
子句加返要形容的【主句部份】進去,會是:【a summer】blazes people。而summer 這裡是noun(吞subject), 所以用which而不用when。