Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
pledge [plɛdʒ] n (promise) обяза́тельство
vt (promise, money, support, help) обяза́ться (perf)
to pledge sb to secrecy брать*(взять*perf) с кого́-н сло́во молча́ть
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.