什麼情况下用am are is was were 什麼时候不用..

什麼情况下用am are is was were 什麼时候不用..
什麼情况下用am are is was were 什麼时候不用..

什麼情况下用am are is was were 什麼时候不用..
第一,后面是动词+ing的形式时
第二,后面是形容词时