As 作为连词有三种用法

1. 表方式： 按照

2. 表原因： 由于、既然

3. 表时间：当….

例句：

1）Do as I told you. 按我说的做。

2) As you had already known Tom, there is no need for instructions. 既然你已经认识Tom了，那就没必要介绍了。

3）We were lying on the beach sunbathing as they were playing volleyball. 当他们在沙滩上打排球时，我们在晒太阳浴。