Some things是指你想说的不确定数量的未被指明的事物.比如： Some things are difficult to understand.
Something 是指某一个不确定（未被指明）的事物. 比如：Something is wrong there.
至于怎么尽快地记住什么时候用哪个,再给你两个例子：
Some things-
A: Hey have you heard Eminem's new song?
B: No I don't really like him. SOME THINGS in his songs are hard to understand.
Something-
A: I heard you broke up with your girlfriend last night, what happened?
B: Yeah SOMETHING just doesn't feel right in our relationship but I don't know what it is.