en_tn_lite_do_not_use/mrk/09/02.md

alone by themselves

The author uses the reflexive pronoun "themselves" here to emphasize that they were alone and that only Jesus, Peter, James, and John went up the mountain. (See: rc://en/ta/man/translate/figs-rpronouns)

he was transfigured

"Transfigure" means for a person's outward appearance to change. This can be stated in active form. Alternate translation: "his appearance had changed" or "he appeared very different" (See: rc://en/ta/man/translate/figs-activepassive)

before them

"in front of them" or "so they could clearly him"