原句
They are strict.
（他们很严厉。）
否定句
They are not strict.
解析
句子里有 be 动词 are，变否定句直接在 are 后面加 not。
规则：
is → is not
are → are not
am → am not
这里主语是 they，be 动词是 are，所以否定就是 are not。