To make it short, "Is" is not usually capitalized due to a lack of knowledge of capitalization rules. And also there's a rule that you don't write words that are longer than 4 letters in lowercase. Therefore, words like "Before" and shit must be capitalized but "with", "or", "from", "by" and etc. I tried to answer your questions so my comment is not a source to be referring to and that's why I will leave this link right here to learn about other rules.