iary_ verbs. The most common are: _do, be, have, may,
must, might, can, shall, will, should, would, could_, and _ought_. Some of
these may be used as principal verbs. A few notes and cautions are added.

_Can_ is used to denote the ability of the subject.

_May_ is used to denote permission, possibility, purpose, or desire. Thus
the request for permission should be, "May I?" not "Can I?"

_Must_ indicates necessity.

_Ought_ expresses obligation.

_Had_ should never be used with _ought_. To express a moral obligation in
past time, combine _ought_ with the perfect infinitive: [I ought _to have
done_ it].

_Should_ sometimes expresses duty: [You should not go].

_Would_ sometimes denotes a custom: [He would sit there for hours].
Sometimes it expresses a wish: [Would he were here!]. For other uses of
_should_ and _would_, see Appendix 60.

+55. Principal Parts.+--The main forms of the verb--so important as to be
called the _principal parts_ because the other parts are formed from them--
are the _root infinitive_, the _preterite_ (_past_) _indicative_, and the
_past participle_ [move, moved, moved; sing, sang, sung; be, was, been].
The _present_ participle is sometimes given with the principal parts.

+56. Inflection.+--As is evident from the preceding paragraph, verbs have
certain changes of form to indicate change of meaning. Such a change or
_inflection_, in the case of the noun, is called _declension;_ in the
case of the verb it is called _conjugation_. Nouns are _declined_; verbs
are _conjugated_.

+57. Person and Number.+--In Latin, or any other highly inflected
language, there are many terminations to indicate differences in person
and number, but in English there is but one in common use, _s_ in the
third person singular: [_He runs_], _St_ or _est_ is used after _thou_ in
the second person singular: [_Thou lovest_].

+58. Agreement.+--Verbs must agree with their subjects in
person and number. The following suggestions concerning
agreement may be helpful:--

1. A compound subject that expresses a single idea takes a singular verb:
[Bread and milk _is_ wholesome food].

2. When the members of a compound subject, connected by _neither ... nor_,
differ as regards person and number, the verb should agree with the nearer
of the two: [Neither they nor I _am_ to blame].

3. When the subject consists of singular nouns or pronouns connected by
_or, either ... or, neither ... nor_, the verb is singular: