I happened to be upstairs at dusk.这里的upstairs是什么词性的?前面有be的.这里的be是助词还是有实义的?如果upstairs还有其他词性,也请帮我造个句好吗?

翻译:I happened to be at the station ,when it happened There happened to be no one in your office when I phoneThere happened to be怎么回事啊` 去掉怎么样 To be or not to be ups to you I ________ out when the telephone rangA happen B happened C happen to D happened to be 请说明为什么? 连词成句：came,you,to,happened,out,when,your,be,house,I,to I happened to be sitting next to an established programming writer/guru,这个怎么翻译? I want to be told all( )A.which happen B.which happened C.that had happened D.that had been happened When you called me this morning ,I happened -------a meeting Ato be having B.to have I happened to be out when Linda called me yesterday 这句话有问题没. the old man happened to be on There ( )nobody in the room.were just happened being were happened happened to be. Peter was happened to be at home that day.为什么是was happened而不是happened I'm sorry that I happened _____ when you came to see me last nightA:be outB:outC:to outD;to be out what happened to him at last __more than i know.A.were B.to be C.are D.is It happened ( ) when I got off the bus in beijingA.to rain B.raining C.rained D.to be raining It happened to be 和There happened to be有什么区别? I happened to check out videos he ____ by his sister at that moment.A.happened to see B.was happened to see C.was happened to be seen D.happened to be seen