Exegesis:

tōn de ochlōn epathroizomenōn ‘when the crowds were increasing,’ or, ‘were crowding together,’ preferably the latter. epathroizomai.

ērxato legein ‘he began to say,’ cf. on 4.21.

genea ponēra estin ‘is an evil generation.’ The repetition of genea is intentional and emphatic.

sēmeion zētei ‘it demands a sign,’ cf. on v. 16.

kai sēmeion ou dothēsetai … ei mē to sēmeion Iōna ‘and no sign will be given except the sign of Jonah,’ cf. on v. 30.

Translation:

Were increasing, preferably, ‘were crowding around Jesus.’

This generation is an evil generation, or, “how evil are the people of this day” (Good News Translation), and cf. on 7.31.

No sign … except, or, ‘no other sign … but,’ ‘the only sign that … is,’ ‘(no more than) one sign…, namely.’

Shall be given. The implied agent is God.

The sign of Jonah, or, where necessary to clarify the relationship between noun and proper name, something like, ‘the sign that (or, a sign like the one that) Jonah gave,’ ‘the one Jonah showed/produced.’ The rendering of sign should bring out the signifying, not the miraculous aspect.

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.