en_tn/gen/38/09.md

he went to his brother's wife

This is a polite way of referring to having sexual relations with her. See how you translated a similar phrase in Genesis 16:2. (See: rc://en/ta/man/jit/figs-euphemism)

he wasted it on the ground

You may need to make explicit that the word "it" refers to Onan's semen. Alternate translation: "he destroyed his semen by having it flow on the ground" or "he did not send his semen into her" (See: rc://en/ta/man/jit/figs-explicit)

on the ground

This is probably a euphemism for any place that it should not have gone. (See: rc://en/ta/man/jit/figs-euphemism)