Quenya Imperativ without free "a"

How could words like "must" or "should" be expressed? Like for instance "you must come." instead of "come!" I mean: how can I tell a person that he/she is forced to do something, but that I am not the one who forces him/her? Could it be usefull to put the imperative particle to the word's end instead of putting it in front of it? Like in cena, tula and tira? Or could maybe the forms with "a" at the beginning and the pronominal suffixes at the end be used to express this status of being forced? Like alye for instance? Would that mean "you must"?

PS. And one more thing: Could I maybe just use "horya-" as "must"? I think that would be much easier.