6,  There are countless people who, ever since their early years, have learned to associate snails with food.
 countless: 无数的，数不尽的 (=numerous)
      who引导定语从句
      associate sth. with sth. 把某物与某物相联系新概念英语

7, Acting on a sudden impulse, I collected several dozen, put them in a paper bag, and took them to Robert.
on a sudden impulse 一时的，冲动的
    Acting on a sudden impulse, he went shopping to buy several dresses.
    Robert was delighted to see me and equally pleased with my little gift. ==Robert was not only delighted to see me, but also pleased with my little gift.
 
8, 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.
happened to do 恰巧，碰巧
     I happened to be out when he called.
     We both happened to be travelling on the same plane.
     happen / chance
     It happened that I was out when he called.
     It chanced that I was out when he called.