... thorough analysis of the errors committed by Vietnamese students of English in using English modal verbs can, could, may, have to and must An analysis specializing in errors in using English modal verb ... the errors in using modal verbs can, could, may, , must and semi -auxiliary verb have to by Vietnamese learners at TL secondary school - Find the major causes and sources of their errors and - ... teaching and learning: can, could, may, have to, and must 1.3 SCOPE OF THE STUDY Within the limitation of a graduation thesis, I would like to deal with modal verbs can, could, may, have to and must...