You are welcome to answer questions posted in the Ask a Teacher forum as long as your suggestions, help, and advice reflect a good understanding of the English language. If you are not a teacher, you will need to state that clearly at the top of your post. Please note, all posts are moderated by our in-house language experts, so make sure your suggestions, help, and advice house the kind of information an international language teacher would offer. If not, and your posts do not contribute to the topic in a positive way, they will be subject to deletion.
You may first try the Grammar Glossary on the left of your post (look up 'modal verbs') I fear this part of grammar is too comprehensive to sum it up in few words.
You can come back here, and try to build some sentence with these verbs so that we can advise you on them.
grammar 
Linear Mode
