1,we must know how to learn engilsh “to learn”;可以换成learning吗

we must know how to learn engilsh “to learn”;可以换成learning吗?
不能换
疑问词除了why其他的都可以直接加to do,这是个短语,不是句子
where to go,when to leave,what to do,whom to ask等等
2,where do they work?"do"可以换成“are"吗?
不可以换成are
work是实意动词,实意动词构成问句、否定句要加助动词do\does\did 而不是is\am \are