en_tn_condensed/psa/078/002.md

open my mouth in parables

The idiom "open my mouth" means to speak. AT: "speak in parables" (See: rc://en/ta/man/translate/figs-idiom)

sing about

"tell." See how "pours out" is translated in Psalms 19:2.

hidden things

If your language has a word for sayings that are purposely difficult to understand, you might use it here.