promise vs troth

promise and troth both are nouns.

Word	Noun	Adjective	Verb	Adverb
promise	Yes	No	Yes	No
troth	Yes	No	No	No

As nouns, troth is a hyponym of promise; that is, troth is a word with a more specific, narrower meaning than promise:

promise: a verbal commitment by one person to another agreeing to do (or not to do) something in the future
troth: a mutual promise to marry

Other hyponyms of promise include oath, parole, word, word of honor, betrothal, engagement, pinning, rain check.

promise (noun)	troth (noun)
grounds for feeling hopeful about the future	a solemn pledge of fidelity
a verbal commitment by one person to another agreeing to do (or not to do) something in the future	a mutual promise to marry

promise (verb)	troth (verb)
make a promise or commitment
promise to undertake or give
make a prediction about; tell in advance
give grounds for expectations