Complex sentence: Etymologically, 是 developed from the meaning of "straight"; in modern Chinese, however, it can be combined with the modifier 的 "de" to mean "yes" or to show agreement.
Split into simpler sentences: Etymologically, 是 developed from the meaning of "straight". Another use of 是 in modern Chinese is in combination with the modifier 的 "de" to mean "yes" or to show agreement.