You use merely to emphasize that a particular amount or quantity is very small.
The brain accounts for merely three per cent of body weight.
大脑仅占体重的3%。
not merely
You use not merely before the less important of two contrasting statements, as a way of emphasizing the more important statement.
The team needs players who want to play for Canada, not merely any country that will have them.
这个队需要的是愿意为加拿大去比赛的运动员,而不仅仅是愿意为任何一个雇用他们的国家去比赛的运动员。