en_tn_lite_do_not_use/mat/17/20.md

For I truly say to you

"I tell you the truth." This adds emphasis to what Jesus says next.

if you have faith even as small as a grain of mustard seed

Jesus compares the size of a mustard seed to the amount of faith needed to do a miracle. A mustard seed is very small, but it grows into a large plant. Jesus means it only takes a small amount of faith to do a great miracle. (See: rc://en/ta/man/jit/figs-simile)

nothing will be impossible for you

This can be stated in a positive form. Alternate translation: "you will be able to do anything" (See: rc://en/ta/man/jit/figs-litotes)