{As they were departing from him} (en t(9369) diach(9372)izesthai
autous ap' autou). Peculiar to Luke and another instance of
Luke's common idiom of en with the articular infinitive in a
temporal clause. This common verb occurs here only in the N.T.
The present middle voice means to separate oneself fully (direct
middle). This departing of Moses and Elijah apparently
accompanied Peter's remark as given in all three Gospels. See for
details on Mark and Matthew. {Master} (Epistata) here, {Rabbi}
( Mr 9:5 ), {Lord} (Kurie,  Mt 17:4 ). {Let us make}
(poi(8873)(936d)en, first aorist active subjunctive) as in  Mr 9:5 ,
but  Mt 17:4  has "I will make" (poi(8873)(935c)). It was near the time
of the feast of the tabernacles. So Peter proposes that they
celebrate it up here instead of going to Jerusalem for it as they
did a bit later ( Joh 7 ). {Not knowing what he said} (m(8820)eid(9373)
ho legei). Literally, {not understanding what he was saying}
(m(885c), regular negative with participle and legei, present
indicative retained in relative clause in indirect discourse).
Luke puts it more bluntly than Mark (Peter's account), "For he
wist not what to answer; for they became sore afraid" ( Mr 9:6 ).
Peter acted according to his impulsive nature and spoke up even
though he did not know what to say or even what he was saying
when he spoke. He was only half awake as Luke explains and he was
sore afraid as Mark (Peter) explains. He had bewilderment enough
beyond a doubt, but it was Peter who spoke, not James and John.