“立誓”常见的英文表达是 “make a vow” 或 “swear an oath” 。
make a vow：更侧重于表达个人郑重地作出承诺或誓言，常带有一种情感上的决心。例如：She made a vow to herself that she would lose weight.（她对自己立誓要减肥。）
swear an oath：语气更为正式、庄重，常用于法律、宗教等严肃场合，表示宣誓、发誓。例如：Newly elected officials swear an oath of office.（新当选的官员进行就职宣誓 。）