as的意思=because,as可以引导一个原因状语从句。
也就是说As there are so many people about, the dogs have to be kept in a special enclosure.=Because there are so many people about, the dogs have to be kept in a special enclosure.
逗号是在从句在前时加上,比如:
When I was having lunch,someone knocked at the door.有逗号
Someone knocked at the door when I was having lunch.无逗号
as的用法如下：as可用作连词，引导比较、时间、原因及方式等四种状语从句，应注意的是，引导的比较状语从句往往有省略；引导的时间状语从句一般用一般现在时而不用一般将来时；引导原因状语从句时与“Because”和“since”引导的从句比较起起来语气最弱。
as作介词，意思时“作为”，“以……身份”。例如：He came to China as a tourist five years ago.
而表示像…一样时，like通常作介词用，而介词后面通常接名词，代名词和动名词。
as in Australia“和在澳大利亚一样”。
as……as
AS +adj(原级)+AS
AS +adv(原级)+AS
as soon as 一……就
as soon as possible 尽可能快地
as early as possible 尽可能早的
as carefully as you can 尽可能认真地
as careful as you can 尽可能认真的
1。as是连词，表示“随着”的意义。lives是life的复数，意义是“生活”。<BR>
2。as是连词，表示“因为，由于”的意义。wanting是现在分词，表示主动意义，wanting to buy cars=who want to buy cars。
参考资料：
http://zhidao.baidu.com/link?url=2W-Gd5hmoWooAe1ilJS8D8Pq7GmcSHJQ7_pSJP0ZZEYiWo55TrcXntU_XDrjx685C9XhjxkFxEmvUgqBlj-bMq祝学习进步
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