1.Were they here yesterday?
When were they here?
They weren't here yesterday.
2.Must he leave early?
Why must he leave early?
He needn't leave early.
其实很简单.一般疑问句就是be动词提前或情态动词提前
特殊疑问句是疑问词加一般疑问句
否定句就直接在be动词或情态动词后加not
must特殊要改成need