这两个都是固定句式.used to do本身从时态上讲,是过去时,指过去常常做某事,强调的是过去做,而现在不做了.这里的to 是动词不定式.如：I used to play chess.我过去常常下棋.
was/were used to doing,当然也有is/are used to doing.统称为be used to doing.这里,to是介词,所以后跟动句词.意思是习惯于做某事.如：I was used to going out when it was raining.我习惯于雨天外出.