1.help to do sth = help do sth 帮着做某事
如：Please help carry this heavy box.
= Please help to carry this heavy box.
请帮忙抬一下这个重盒子.
当然,如果是在被动语态中,则这个to不能省略.
2.单独的短语help doing sth是不存在的.
但是有：can't/couldn't help doing sth
= can't/couldn't help but do sth
= can't/couldn't stop doing sth
意思是“禁不住/忍不住做某事”
如：I can't help crying.
= I can't help but cry.
= I can't stop crying.
我禁不住/忍不住哭了.