都表示承诺保证的含义。
1. pledge 用法更为正式，用于郑重场合
- The French president is pledging $150 million in French aid next
year.
- The president pledged himself to increase taxes for the rich but
not the middle classes.
2. promise 更为口头随便
- Promise me you will not waste your time.
- I‘ll call you back, I promise.
同义词解析 (pledge promise),布布扣,bubuko.com
同义词解析 (pledge promise)
原文：http://www.cnblogs.com/myprogram/p/3775301.html
