属正式用词，主要用于肯定句或疑问句，一般不用于否定句(否定句可用either)，它在句中通常放在实义动词之前，特殊动词 (动词be、情态动词、助动词)之后，有时也放在整个谓语前面或放在句首或句尾 (较少见，且多半是因为强调之故)。

Also

also是用在肯定句里面，它表示的是两个人干了同样的事，或者两件事是同一个人干的，also一般是位于be动词，助动词，情态动词的后面，但在行动词的前面。比如像：

He has been to Beijing. I have also been there.

He is good at English. He is also good at Japanese.

不过不也有一种特殊情况，那就是在表示强调的时候，also也是可以放在be动词，助动词，情态动词的前面。比如像：

I can also do it.

I also can do it.

As well

这个一般在口语中用的比较多，也是用于肯定句，和too可以进行互换，通常是位于句末。比如像：

She not only sings,

She plays the piano as well.

Too

Too和as well的用法基本一样，它比also会更加的通俗易懂，和also，as well都可以进行相互的替换，可以放在句子的中间，也可以放在句末位置。比如像：

Xiao Li went to Beijing and Xiao Zhao, too, went to Beijing.

He studies hard and I study hard, too.

Either

这个与前面三个不太一样，它是用在否定句里面的，并且还只能放在句末。比如像：

He didn't know it. I didn't know it either.

I can't speak French and can't write it, either.