Back 20 years ago now, I took a natural language processing class.  Overall the class was excellent and it really taught me a lot about how languages work.  One handout in particular was useful to me.  The instructor had us do simple verbs at first, and that worked great, but then we had to start parsing helper verbs and the instructor gave us a sheet that he had copied out of a book: Summary of the verb system in A Student's Grammar of the English Language, by Greenbaum and Quirk, Longman Publishers, 1990.

CS 469B/569B
Natural Language Processing - 1992
Cem Bozsahin

English Verb System

English verbs can be characterized by

1. Form
2. Tense
3. Auxiliary (optional)
   

constructions.  The table below shows the classifications based on form and tense.

Form Name	Regular Example	Irregular Example
base	work	go
s	works	goes
ing	working	going
past	worked	went
past-participle	worked	gone

Tense Name	Number of Subjects	Verb Form	Example
present	3rd person singular	s	he sings
present	others	base	they sing
past	all	past	he sang

Auxiliary constructions may proceed a verb.  The first auxiliary (if present) carries the tense in English.  Their ordering is as follows (parentheses mean optional items):

(modal) (perfect) (progressive) (passive) main-verb

For example, he would have sung has two auxiliaries (would and have), the main verb is sing and the tense is past (would is the past form of will).

If no auxiliary is present, the verb carries the tense (e.g., he sang).  Below is a list of auxiliary verbs:

type	base form	past form	past-participle form
modal	will, can, shall, may, must	would, could, should, might
perfect	have, has	had
progressive	be (is, am, are, be)	be (was, were, be)	been
passive	be (is, am, are, be)	be (was, were, being)	been

The combinations of perfect and progressive is called the aspect of the main verb.  Below is the list of possible combinations:

aspect	comment	example
simple	no perfect or progressive	he sang
perfect	perfect only	he has sung
progressive	progressive only	he was singing
perfect-progressive	both perfect and progressive	he has been singing

The presence of an auxiliary verb signals the form of the next verb as well.  In the table below, a verb type in the first column is supposed to be followed by a verb form in the second column:

aux verb	form of the next verb	example
modal	base	he will go home
perfect	past-participle	he has written two papers
be(progressive)	ing	he was singing a song
be(passive)	past-participle	it was brought to Mary by John
do	base	I did go to the theater
no aux	base or ing	to err is human (base) playing piano is fun (ing)