1.be useful to------- 2.be helpful to------3.be important to

e useful to do sth
it is helpful to do sth.做……是有好处的
It is helpful for sb.to do sth.某人做……是对他有好处的
这两个都是it 做形式主语
helpful 是表示“有好处”,而不是“有助的”
it is helpful in doing sth.
sth.is helpful to sth./ doing sth.前面的sth.一般是具体的某个事物
be important to do sth ,be important for sb to do sth
be necessary to do sth