A verb is a word which, in association with a subject, affirms, negates or questions a situation. The situation may be:
(1) A state, something that is conceived of as existing, rather than happening, and as being homogeneous, continuous and unchanging throughout its duration (verbs: be, seem);
(2) a process, a dynamic happening extended in time (verbs: change, develop);
(3) an event, a momentary happening (verbs: explode, sneeze);
(4) an action, a happening under the control of an agent (verbs: run, write).
Differences in structure with affirmation and interrogation suggest a practical division of verbs into four classes:
Primary verbs consist of three verbs: be (forms: be, am, is, are, was, were, been, being); have (forms: have, has, had, having) and do (forms: do, does, did, done, doing). They operate as auxiliary verbs and, when used in conjunction with full verbs, they enable the formation of negative, interrogative and emphatic forms (do) the perfect aspect (have) and the progressive aspect and the passive voice (be) These three verbs also operate as full verbs, though be and have are formally different from the other full verbs.
This class (also known as lexical verbs or content verbs) includes all verbs in English other than the modals, and the primary verbs used as auxiliaries. They have a 'meaning' (as opposed to a grammatical function, and refer to events (for example, explode), actions (run), states (seem) or processes (change). New full verbs appear frequently, (recent examples being google, text, SMS).
The modals consist of nine verbs: shall, should; will, would; can, could and may, might (need, ought and dare can also sometimes function as modals). These operate (though in a different manner from primary verbs) as auxiliary verbs: when used in conjunction with main verbs they enable the communication of some degree of modality, which communicates the speaker's idea of such concepts as the ability, advisability, certainty, determination, expectation, inference, intention necessity, obligation, permission, possibility, probability, volition, of the situation referred to by the full verb.
Secondary auxiliary verbs
- get, formally a full verb, is used as an alternative to be for the passive voice;
- go, also formally a full verb, the durative form of which is used for the prospective aspect;
- used (to) which has some of the formal characteristics of a modal, and which is used for past habitual forms.
Tense, aspect and mood
These concepts are not as clearly differentiated as they are in some languages, and writers on the subject do not always use the words in exactly the same way.
See main article Tense
Tense is generally defined in ways such as: the relationship of the form of the verb and the time of the action it describes. If we confine ourselves to the form, then English has only two simple (i.e. unmarked for any other aspect) tenses, as shown in the pairs play/played, work/worked, want/wanted, wash/washed, sing/sang. The first of these forms is traditionally called the Present Simple, the second the Past Simple. Despite the names, English tenses often bear little relationship to time. The present tenses are often used to describe constant or repeated situations. The past tenses are often used for hypothetical or counterfactual situations. Both tenses can be used for situations in past, present, future and general time. There is no future tense as such in English, though there are several ways of expressing the future.
See main article Aspect
Aspect is generally defined in ways such as: a grammatical category which deals with how the event described by a verb is viewed, such as whether it is in progress, habitual, repeated, momentary, etc..
Some writers use the word tense for such forms as I am/was working and I have/had worked. These, are better considered as aspects. The form shown in I am/was working is traditionally known as the progressive or (continuous). The form shown in I have/had worked, is traditionally known as the Perfect.
See main article Mood
Mood is the word used for the form of the verb which shows the speaker’s attitude to the action denoted, for example, whether it is desired or considered to be uncertain or unreal. The three moods generally distinguished in English are theindicative, the subjunctive and the imperative.
Person and number
The person of a verb is the deciding factor in the choice of subject pronouns and, in some languages, the inflectional endings of the verb. They are:
The first person. The subject of the verb is the speaker(I), or includes the speaker (we). In English, only the verb be has a distinct form for this person (am, singular; are, plural) in the present tense; in all other verbs, the first person singular form is identical to the bare infinitive.
The second person. The subject of the verb (you) is the person or people addressed by the speaker. In English, only the verb be has a distinct form for this person (are) in the present tense; in all other verbs, the second person singular form is identical to the bare infinitive.
The third person. The subject of the verb is the person, people, thing or thing (he, she, it, they, noun. etc) referred to by the speaker. In English, only the verb be has a distinct form for this person (is, singular, are, plural) in the present tense. In all other verbs, the third person plural form is identical to the bare infinitive. In full verbs, the third person singular form adds -(e)s; the third person singular form of modal verbs has no ending. See Pronunciation of the morpheme “-s”.
A special case - the nefarious 'is'
The simple word “is” has been noted since ancient times for its slipperiness. Three of its uses — the genus-species “is”, the predicative “is”, and the “is” of identity — are well discussed in an article by Mary Kathleen Roberts. 
- Lyons, John(1977) Semantics, Cambridge: CUP
- "The OEC: Facts about the language" Oxford Dictionaries. Retrieved 30th September 2012.
- Richards , Jack C, Platt, John and Platt, Heidi (1992) Longman Dictionary of Language Teaching & Applied Linguistics (2nd Edition), Harlow: Longman
- Trask, R. L. Mind the Gaffe (2001), ISBN 0-14-051476-7
- Roberts, M. K. The Nefarious “Is” (2013) in K. E. Davis, R. Bergner, F. Lubuguin, & W. Schwartz (Eds.), Advances in Descriptive Psychology (Vol. 10, pp. 267-277) Available in Scribd.