en_tn_condensed/mat/17/19.md

we

Here "we" refers to the speakers but not the hearers and so is exclusive. (See: rc://en/ta/man/translate/figs-exclusive)

Why could we not cast it out?

"Why could we not make the demon come out of the boy?"

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/translate/figs-simile)

nothing will be impossible for you

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