11、For years he has been asking me to collect snails from my garden and take them to him.

12、The idea never appealed to me very much, but one day, after a heavy shower, I happened to be

walking in my garden when I noticed a huge number of snails taking a stroll on some of my prize plants.

The idea never appealed to me very much.=The idea never interested me very much.

happen to do 偶然, 碰巧

-- I happened to out when he called.

-- We both happened to be traveling on the same plane. happen / chance

-- It happened that I was out when he called.=It happened that I was out when he called.

When 表示正在这时突然(常与过去进行时、过去完成时连用, 位于句中)

-- He was having a bath when the telephone suddenly rang.

-- We were about to start when it rained. taking a stroll 现在分词做定语修饰 snail snails taking a

stroll=snails that were taking a stroll