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.