应该是看介词前面的动词吧，比如：I was at home watching TV.可以转换成：I was watching TV at home.这就随be动词的用法：be doing sth.

如果是：I go to school to learn many uesful things.可以转换成：I go to learn many uesful things in school.这就随一般动词+to do sth.的用法吧。