My baseball is under the table.（对under the table 提问）
under the table是地点 -->Where is your basketball?
你的篮球在哪儿?
My math book is on the desk.(改为一般疑问句)
助动词提前-->Is your math book on the desk?
你的数学书[是不是]在桌面上?
They are some good dictionaries.（改为单数句）
it is a good dictionary. 这是一本好的词典.
Under the bed is a red ruler.（改为同义句）
a red ruler is under the bed.倒装句顺过来变成主谓顺序.
有一把红色的尺子在床底下.