彼らがけんかをやめてくれればいいのに。

they (usually male); them

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

quarrel; brawl; fight; squabble; scuffle; argument

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to stop (an activity); to cease; to discontinue; to end; to quit; to cancel; to abandon; to give up; to abolish; to abstain; to refrain

good; excellent; fine; nice; pleasant; agreeable; sufficient (can be used to turn down an offer); ready; prepared; profitable (e.g. deal, business offer, etc.); beneficial; OK

although; when; and yet; despite this; in spite of; even though; but even so; but even then; however; nevertheless; for all that; notwithstanding that; while; if only; I wish; I tell you; you should do; in order to