1. 只不过;仅仅
    You use merely to emphasize that something is only what you say and not better, more important, or more exciting.

merely

    e.g. Michael is now merely a good friend...
           迈克尔现在仅仅是个不错的朋友而已。
    e.g. Francis Watson was far from being merely a furniture expert...
           弗朗西斯·沃森远远不止是个家具行家。

2. 只不过，仅仅(用于强调数量之小)
    You use merely to emphasize that a particular amount or quantity is very small.

    e.g. The brain accounts for merely three per cent of body weight.
           大脑仅占体重的3%。

3. 不仅仅
    You use not merely before the less important of two contrasting statements, as a way of emphasizing the more important statement.

    e.g. The team needs players who want to play cricket for England, not merely any country that will have them...
           球队需要的是想为英格兰而战的板球选手，而不是愿为任何国家打球的选手。
    e.g. His were not merely crimes of theft but of violence against elderly people.
           他所犯的不止是偷窃罪，而是针对老年人的暴力犯罪。