 Let's now have a look at difficulties that students have when learning modal auxiliary verbs. They present specific problems for both teachers and learners, quite apart from the numerous difficulties associated with the myriad of usages that there are, and the combination of which modal verbs can be used for which usage. There are difficulties also with the structure of sentences, for example, forming a negative. Usually we use the auxiliary verb do with most verbs to form a negative, but with modal auxiliary verbs we don't. So for example, I don't like swimming, do not, with the verb like. When modal auxiliary verbs are initially introduced to students, they may find it puzzling that we don't use this structure, instead we use this structure. Similarly, another difficulty is when the auxiliary verb do is used for questions. This isn't the case with modal auxiliary verbs. Do you can swim is incorrect, as you create questions by inversion. Can you swim? Another fundamental problem for students learning English is forming positive sentences in third person singular. He likes swimming, and he can swims. He swims is obviously correct, but the tendency may be to extend that to he can swims. The S is omitted here, as the main verb is not conjugated when used with modal auxiliary verbs. So, he can swim.