Sounds that change the spelling
Ten minutes with the two places pronunciation actually changes Serbian spelling: a voiced consonant that meets a voiceless one across a morpheme boundary gets written the way it sounds (потпис), while a normally voiced consonant at the end of a word keeps its letter no matter what (друг). Meet two more everyday phrases, hear the grammar lesson's own потпис sentence said aloud, and build the difference into muscle memory.
The part of the grammar page this lesson drills — read it here, then use it.
Rule 3 - some consonant changes are written, some are not
When a voiced and a voiceless consonant meet at a morpheme boundary, Serbian spelling changes to match the pronunciation: под- (under) + -пис (writing) is written потпис (signature), with д devoiced to т in the spelling itself - the assimilation is written into the word, not merely pronounced.
On the other hand, Serbian does not mechanically devoice a consonant just because it sits at the end of a word. A word like друг (friend) keeps a normally voiced г in careful Serbian speech and always keeps it in writing - do not automatically turn final б/д/г/ж/з into п/т/к/ш/с out of habit.