"Is tomorrow colder than today?"
Better to say, "Will tomorrow be colder than today?"
This is a problem we keep seeing in this Duolingo program; the English translations are very often poor English, and we often face the frustration of renditions with better English, being rejected. Apparently this course was made by Chinese-speakers, without adequate input from nativ English-speakers. What i suggest is that this course's translations be reviewed by English-speakers; and that any expansions of this course be done in consultation between English-speakers and Chinese-speakers, so that both the English and the Chinese are good. Same with Hindi for English Speakers, which is currently very short (only 32 lessons, compared to ~100 in other Duolingo courses, so expanding it should be a priority); such expansion should be done in consultation between English-speakers and Hindi-speakers.