Our teacher went to Beijing to attend a meeting."to attend a

我们老师去北京开会了.对去开会部分提问,也就是说你们老师去北京干什么了?也就是为什么去北京?可以用why,也可以用what for.
所以应该是Why did your teacher go to Beijing?或者What did your teacher go to Beijing for?
这里跟did还是is没有关系,what for,本身就可以表示原因,至于要使用哪个助动词,是did还是is,要根据句子具体情况来确定.原句中动词是went,所以应该是一般过去时,提问就应该用did...go.