Exegesis:

hina mēpote … pantes hoi theōrountes arxōntai autō empaizein ‘lest surely … all the onlookers began to make fun of him,’ syntactically a final clause in the subjunctive, dependent upon v. 28, describing what would happen if the answer to the question of v. 28 were not in the negative. hina mēpote is somewhat stronger than hina mē. mēpote 2.b.α and on 12.58. For theōreō cf. on 10.18.

empaizō with dative ‘to mock,’ ‘to ridicule,’ ‘to make fun of.’

thentos autou themelion kai mē ischuontos ektelesai ‘when he has laid the foundation and (then) is unable to finish,’ temporal clause in the genitive absolute. The aorist tense of thentos denotes a fact preceding that denoted by ischuontos in the present tense. For themelios and ischuō cf. on 6.48.

ekteleō ‘to complete,’ ‘to finish.’

(V. 30) houtos ho anthrōpos ‘this man,’ with a note of contempt, cf. Plummer.

ērxato oikodomein kai ouk ischusen ektelesai ‘began to build but was unable to finish,’ aorist tense expressing that the onlookers regard the story as an event of the past.

Translation:

Changing the structure of this sentence one may say, ‘if not (or, if he does not do so), it may happen (that) he lays the foundation but is not able to finish its-building; then all who see….’

To lay a foundation, or, ‘to place/make/build the base, or, the supporting stones/beams, or, what is to support it (i.e. the tower).’

Finish, or, ‘finish it (i.e. the tower).’ The verb is synonymous with ‘complete’ in v. 28.

To mock, or, ‘to make fun of,’ ‘to laugh at.’ Some languages prefer to use here a distinctive term in accordance with the specific kind of mockery indicated in the next verse, e.g. ‘to jeer/taunt.’

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.