Unless ye utter speech easy to be understood (ean mē eusēmon logon dōte). Condition of third class again (ean and aorist subjunctive). Eusēmon (eu, well, sēma, sign) is old word, here only in N.T., well-marked, distinct, clear. Good enunciation, a hint for speakers.
Ye will be speaking into the air (esesthe eis aera lalountes). Periphrastic future indicative (linear action). Cf. aera derōn (beating the air) in 1Co 9:26. Cf. our talking to the wind. This was before the days of radio.