Based on syntactic criteria English verbs can be divided into two main categories--a very large and open one of lexical verbs (e.g. see, walk, run, etc.) and a much smaller and closed one of auxiliary verbs (e.g. shall, should, can, will, etc.). The syntactic criteria are that lexical verbs require do in questions, negative sentences and as a pro-form after so. Consider, for example, the lexical verb see and the auxiliary verb should in the following sentences:
lexical verb
John saw the play
Did John see the play?
John did not see the play.
John saw the play, and so did Bill.
auxiliary verb
John should see the play.
Should John see the play.
John should not see the play.
John should see the play, and so should Bill.
The auxiliary verbs include do, have and be, and the modal verbs can, could, may, might, shall, should, will, would, used to, must, ought to, need, and dare. (Need and dare may behave syntactically like auxiliary or lexical verbs.)
The errors discussed in the following sections involve the wrong choice or omission of modal or auxiliary verbs. Since these are errors of meaning or usage rather than of syntax, it is necessary to include the discussion of other verbs which are similar or contrast in meaning but are not auxiliary verbs according to the criteria explained above. These include: have to, be supposed to, had better, let, and be to. Quirk et al. (1972: §3.8) refer to some of these as semi-auxiliary verbs. The construction be to + infinitive is discussed separately (cf. 2.7) because it is a particularly frequent source of error, though it too can be considered a semi-auxiliary verb and shares certain semantic features with the modals.
Because the choice of verb, even among this relatively limited set of auxiliary and auxiliary-like verbs. is considerably more arbitrary than, for example, the choice of tense, it would be pointless to try to classify these errors as violations of individual lexical or semantic rules, since in most instances there is no single lexical rule specifying the use of a particular verbal construction, even taking context into consideration; i.e. there is almost always more than one correct alternative. Therefore, as in the case of prepositions (cf. 4.2), the erroneous sentences are classified here according to the form which is misused; in one error type the modal verb is omitted altogether.