There are dedicated apps like GDZ for Grades 1-11 that host databases of correct answers for popular textbooks, including the Buneev series.
Many online "Reshebnik" sites categorize answers by textbook author, grade, and exercise number. gdz po russkomu iazyku shkola 2100 3 klass
Roots, prefixes, suffixes, and endings; word-formation processes. There are dedicated apps like GDZ for Grades
Types of sentences by purpose and intonation, primary and secondary members of a sentence. and exercise number. Roots
Parents who may not remember specific grammar rules can use the guide to explain difficult concepts to their children.