彼らはきっと恋におちる。

they (usually male); them

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

surely; undoubtedly; almost certainly; most likely (e.g. 90 percent); sternly; severely; having no slack; rigid; stiff; tight; suddenly; abruptly; instantly

(romantic) love

at (place, time); in; on; during; to (direction, state); toward; into; for (purpose); because of (reason); for; with; by; from; as (i.e. in the role of); per; in; for; a (e.g. "once a month"); and; in addition to; if; although

to fall down; to drop; to fall (e.g. rain); to sink (e.g. sun or moon); to fall onto (e.g. light or one's gaze); to be used in a certain place (e.g. money); to be omitted; to be missing; to decrease; to sink; to fail (e.g. exam or class); to lose (contest, election, etc.); to crash; to degenerate; to degrade; to fall behind; to become indecent (of a conversation); to be ruined; to go under; to fade; to come out (e.g. a stain); to come off (e.g. makeup); to be removed (e.g. illness, possessing spirit, name on a list); to fall (into someone's hands); to become someone's possession; to fall (into a trap); to fall (for a trick); to give in; to give up; to confess; to flee; to fall; to be defeated; to surrender; to come to (in the end); to end in; to fall (in love, asleep, etc.); to swoon (judo); to consent; to understand; to crash; to freeze; to die; to move to the depths