”是我们开始做家庭作业的时候了.“
It's time for sb.to do sth.=It's time that sb.should do sth.=It's time that sb.did sth."是某人做.的时候了"这里是虚拟语气.
set about doing ”开始/着手做.“doing是动名词结构,做介词about的宾语.
对于问题补充：
It's time that we set about doing our homework.＝It's time for us to do our homework.=It's time that we (should) set about doing our homework.＝It's time that we should do our homework.=It's time that we did our homework.
It's time that we set about doing our homework.中的set 不是动词原型而是过去式形式,只不过set的过去式也是set.