as ... as possible

1. 尽可能; 愈……愈好

Come as soon as possible.

尽快来。

do one's possible

1. 尽力

He had done his possible though he failed at last.

他虽然最后失败了, 但已经尽了力。

if possible

1. 如有可能

I'll help him if possible.

如有可能我会帮助他的。