they are talking about their homework(改为否定句)？

They are talking about their homework.的否定句是They are not talking about their homework.

说明：are talking是现在进行时，即Be动词+现在分词。只需在be动词后面+not，即可变成否定句。又如：

肯定句：He is reading a book.

否定句：He is not reading a book.

they are talking about their homework.他们正在讨论家庭作业。这里有be动词are,直接在are后面加一个not就变成了否定句。

they are not talking about the homework他们不是正在讨论家庭作业。

They are talking about their homework

他们正在谈论他们的作业

改为否定句

就是

Negative sentence

They are not talking about their homework

They are not talking about their homework.

进行时态的否定句直接在助动词be的后面加not

they are not talking about their homework.
一般现在时：（1、现在的状态。2、经常或习惯性动作。3、主语所具备的性格和能力。4、真理。）
1、标志：often（经常）,usually（通常）,sometimes（有时）,always（总是）,never（从不）,
on Sundays（在星期天）, every day/month/year（每一天/月/年）
2、结构：
（1）主语+连系动词be(am/is/are)+名词/形容词/数词/介词短语/副词等做表语表状态。
（2）主语（非第三人称单数）+行为动词原形+其他 （用助动词do 帮助构成否定句、一般疑问句和特殊疑问）
（3）主语（第三人称单数）+行为动词的第三人称单数+其他（用助动词does 帮助构成否定句、一般疑问句和特殊疑问句）
行为动词第三人称单数加-s的形式
1.- s
2. 辅音+y: study-studies
3.以s,x,ch,sh结尾watch-watches  teach-teaches
4特殊have-has do-does go-goes