man needs water to drink,i want something to drink.would you like something to eat?这三个句子能不能换成这样：man needs to drink water.i want to drink something would you like to eat something.

可以，只是句子结构不同而已

不能 第一句人需要喝水，强调喝的是水
第二句 would you like something to eat？你想要吃点什么吗？强调的是吃的东西

可以的，但就是意思不同：
man needs water to drink, 人需要喝水，
i want something to drink. 我需要喝点什么。
would you like something to eat？你想要吃点什么吗？
man needs to drink water. 人需要喝水。
i want to drink something 我需要喝点什么
would you like to eat something.你想要吃什么吗？

可以的，意思基本一样，侧重点有所不同而已。

可以换,意思稍不同
man needs water to drink,人需要喝水,强调喝的是水
i want something to drink.我需要喝点什么.强调是喝的东西
would you like something to eat?你想要吃点什么吗?强调的是吃的东西
man needs to drink water.人需要喝水.强调喝水这个动作
i want to drink something 我需要喝点什么,强调喝这个动作
would you like to eat something.你想要吃什么吗?强调吃这个动作
希望对你有所帮助.

完全可以，只是man needs water to drink, i want something to drink. would you like something to eat 这些句子中的to do 都是作sth 的定语

man needs to drink water. i want to drink something would you like to eat something.
这些句中的to do sth 都是动词的宾语。

能换，而且后面那样说更好些。