merely只能做副词用
only可以做形容词、副词和连词等等
意思也多很多
merely
adv.
仅仅；只不过
I merely looked at the chocolate; I did not eat it.
我只不过看了看巧克力,我没有吃.
They are not merely content to fill the stomach.
他们不仅仅满足于填饱肚子.
You don't have to be angry. He merely wanted to know the truth.
"你不必太生气了,他只不过是想知道点事实真相."
only
adj.（形容词）
Alone in kind or class; sole.
唯一的：在种类或级别中与众不同的；独特的
Standing alone by reason of superiority or excellence.
出众的：因杰出和优秀而突出的
adv.（副词）
Without anyone or anything else; alone:
单独地：没有其它别的事物；独自地：
room for only one passenger.
单人客房
At the very least:
仅仅：
If you would only come home. The story was only too true.
只要你肯回家来.这故事是千真万确的
And nothing else or more:
只有,仅仅：
I only work here.
我只在这里工作
Exclusively; solely:
专门地,唯一地：
facts known only to us.
这些事只有我们才知道
In the last analysis or final outcome:
最后的分析,最终结果：
actions that will only make things worse.
最后使事情更糟糕的行为
With the final result; nevertheless:
最终结局,仍然：
received a raise only to be laid off.
加薪的结果只是被派遣而已
As recently as:
最近地：
called me only last month.
上个月刚通知我
In the immediate past:
刚才：
only just saw her.
刚才还看见她了
conj.（连接词）
Were it not that; except.
假如；除非
With the restriction that; but:
条件是,但是：
You may go, only be careful.
你可以去,但要小心
However; and yet:
然而,毕竟：
The merchandise is well made, only we can't use it.
这种商品做工精致,然而我们不能用
