1.if 作为"是否"的意思时,常用在口语当中,引导疑问从句,如 DO do you know if xiaoli is at home?你知道小李是否在家吗?
2．作为”假如”的意思时,引导的是条件从句,用于虚拟语气当中,如；If it should be necessary,i could come at six.假如有必要的话,我可以六点钟来．