Exegesis:

ou gar ēdei ti apokrithē ‘for he did not know what he should answer’ (or ‘say’) (for this grammatical construction cf. ti phagōsin ‘what they should eat’ 6.36; ti aiteisthe ‘what you are asking for’ 10.38; ouk ēdeisan ti apokrithōsin autō ‘they did not know what they should answer him’ 14.40).

ekphoboi gar egenonto ‘for they became terrified,’ or ‘for they had become terrified’ (rather than ‘were’).

ekphobos (only here in Mark) ‘terrified,’ ‘very afraid.’

Translation:

The syntactic problems of this verse are complex because of the two uses of for. The first introduces the reason for the preceding action, namely, Peter’s remark, and the second indicates why Peter did not know what to say. In some languages this distinction may be indicated only by expanding the first causal conjunction, e.g. ‘the reason he said this was that he really didn’t know what to say, for they….’ In Navajo the sequence of clauses is shifted ‘since they were exceedingly afraid, he did not know what to say.’

They must be translated in such a way as to refer to the disciples, not to the entire group nor to the immediately preceding third person plural pronoun, they in verse 4, which refers to Elijah and Moses.

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .