en_tn_lite_do_not_use/rom/14/14.md

I know and am persuaded in the Lord Jesus

Here the words "know" and "am persuaded" mean basically the same thing; Paul uses them to emphasize his certainty. Alternate translation: "I am certain because of my relationship with the Lord Jesus" (See: rc://en/ta/man/jit/figs-doublet)

nothing is unclean by itself

You can translate this in a positive form. Alternate translation: "everything by itself is clean" (See: rc://en/ta/man/jit/figs-doublenegatives)

by itself

"by its nature" or "because of what it is"

Only for him who considers anything to be unclean, for him it is unclean

Paul implies here that a person should stay away from anything that he thinks is unclean. You can make this explicit in your translation. Alternate translation: "But if a person thinks something is unclean, then for that person it is unclean and he should stay away from it" (See: rc://en/ta/man/jit/figs-explicit)