Now Susan is very happy. （首位）
Susan is now very happy. （中位）
Susan is very happy now. （末位）

Bill never went abroad. （在行为动词前）
Bill has never gone abroad. （在第一个助动词后）
Bill is never at home these days. （在系动词后）
属于这类中位的副词常见的还有almost, already, always, frequently, nearly, occasionally, often, sometimes, usually等。

never放首位时，主谓要倒装。
Never has Bill gone abroad.
often被quite或very修饰时，可放在末位或首位。
We've been there quite often.
Very often it rains here in December.

tomorrow, then, afterwards, eventually, lately, today等可放在首位或末位。late一般放在末位；yet一般放在末位和否定句连用；still中位，常用于肯定句；just中位，常和完成时连用。放在中位的还有recently, soon等。

副词不能在动词和宾语之间，若宾语较长，可把副词放在动词前。
Children are playing outside.
Bill speaks French well.
Bill secretly decided to solve the question by himself.

too, quite等程度副词，总是放在所修饰的词之前。但enough要放在所修饰的词之后
This box isn't too heavy.
He didn't run quickly enough.

They played fairly well there yesterday. （程度-状态-地点-时间）