it is time that we should do something for our country.关系词为何

It's time + that 是固定句型,that可省略,time可由high,about修饰,从句中的谓语动词有两种形式,或者用动词的过去式(be用were)；或者用shoud + 动词原形（should不能省略).此句型为虚拟语气句型,表示“现在某人该做某事了”.含有该事早该去做而未做,现在去做似乎都为时已晚的意思.