这页要记住一个大规则：be 动词、情态动词都可以直接加 not 变否定；实义动词要靠 do 家族帮忙，过去时用 did，现在时用 do / does。

例句：am not / is not / are not；was not / were not；cannot / should not / must not；do not / does not / did not。