Choose the right grammatical form in each gap to complete the text 1. (Basic, Basically, Basical, Basics), we use a root, add or change a part and get a new word.
2. It's useful to know the (mean, meanings, meaningful, meaningless) of some prefixes.
3. For example, we (knowledge, know, knowing, unknow) that such prefixes like un-, il-, im-, ir-, etc. usually give the oppusite meaning