I decide to do my homework.否定句

怎么说呢，换个词解释就清楚了，例如：

I tell him to do his homework. 我叫他做作业（暂不考虑时态）。

有两种否定：I tell him not to do his homework.和 I don't tell him to do his homework.

区别除了not位置不同，就是意思了。前者“我让他别做作业”，后者“我没让他做作业”。听中文应该能听出有区别吧。

所以说这个题目出的很不好，两种否定都可，建议你选第一种 （don't decide to do my homework）

I don't decide to do my homework
我没有决定要写作业 (到底写不写还未定)

I decide not to do my homework.
我决定不写作业 (已经决定不写了)

I decide to do my homework.（我决定写作业)否定句 的答案应该是后者：
I decide not to do my homework.

前面的意思大概是说话时不想写,过后可能会写,
后面是已经决定不写了。