A great many people,when they speak of home,tend to associate it with a certain atmosphere,

这句话里tend是谓语动词,根据句意是很多人,当然用原形.
when they speak of home 只是插入的状语,你可以把它调到句尾,A great many people tend to associate it with a certain atmosphere when they speak of home,这样就好懂了