Exegesis:
kai idou ‘and behold,’ emphatic introduction of the subsequent account of the boy’s illness, cf. on 1.20.
pneuma lambanei auton ‘a spirit takes, or, seizes him,’ i.e. from time to time, not permanently.
kai exaiphnēs krazei ‘and shouts suddenly.’ Subject of krazei is either the spirit through the boy’s voice (cf. New English Bible), or the boy (cf. Revised Standard Version), probably the former. exaiphnēs also 2.13.
sparassei auton meta aphrou lit. ‘convulses him with foam,’ i.e. so that he foams.
sparassō ‘to convulse,’ ‘to pull to and fro.’
aphros ‘foam.’
kai mogis apochōrei ap’ autou suntribon auton ‘and it withdraws from him (only) with difficulty, mistreating him,’ suntribon auton refers to an act which is simultaneous with apochōrei (cf. Willibrord) and it is best understood as supplementing mogis.
mogis ‘with difficulty,’ ‘with toil and pain.’
apochōreō ‘to go away,’ ‘to withdraw.’
suntribō ‘to mistreat,’ ‘to wear out,’ ‘to bruise.’
Translation:
For spirit, i.e. ‘evil/unclean spirit,’ ‘demon,’ see on 4.33, 35f; for seizes him see on 8.29.
Convulses — the rendering may make us of an idiom for a fit or convulsion, such as ‘cut nose’ (Ekari), or for some of its characteristics, such as spastic movements of legs, arms, and/or body, cf. e.g. ‘causes-him-to-be-contracted’ (Batak Toba), ‘torments him till he jerks’ (Balinese), ‘causes-him-to-turn-and-toss restlessly (lit. as-a-sun-hat)’ (Toraja-Sa’dan), ‘causes his body to break’ (Sranan Tongo).
He foams, or ‘his mouth is (or, lips are) foaming’ (Tae,’ Batak Toba; Toraja-Sa’dan), “He foams at the mouth” (An American Translation), ‘the foam is on his mouth’ (Nieuwe Vertaling).
Shatters him, and will hardly leave him, or, closer to the Greek word order, ‘leaves him only with difficulty, wearing him out, or, and meanwhile it torments him horribly’ (cf. Translator’s New Testament, Willibrord); or shifting him from the act to the resulting state, ‘when it leaves him at last he is wholly broken,’ ‘it leaves him only when (or, it does not leave him before) he is totally worn out.’ In this context to leave somebody is virtually synonymous with “to come out of somebody”, as used in 4.35 (which see).
Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.