The dog trots_.

The s-form is found in the third personal singular of the present
tense. In other tenses, if present at all, the s-form is in the
auxiliary, where the present tense of the auxiliary is used to
form some other tense of the principal verb. Examples: _He has_
(present tense), _He has gone_ (perfect tense), _He has been seen_.

Some verbs have no s-form; as, _will, shall, may_. The verb _be_
has two irregular s-forms: _Is_, in the present tense, and _was_
in the past tense. The s-form of _have_ is _has_.

52. The past tense always stands alone in the predicate; i. e., IT
SHOULD NEVER BE USED WITH ANY AUXILIARIES. To use it so, however,
is one of the most frequent errors in grammar. The following are
past tense forms: _went, saw, wore, tore_. To say, therefore, _I
have saw, I have went, It was tore, They were wore_, would be grossly
incorrect.

53. The third principal part, the past participle, on the other
hand, CAN NEVER BE USED AS A PREDICATE VERB WITHOUT AN AUXILIARY.
The following are distinctly past participle forms: _done, seen,
sung_, etc. One could not then properly say, _I seen, I done, I
sung_, etc.

The distinction as to use with and without auxiliaries applies, of
course, only to irregular verbs. In regular verbs, the past tense
and past participle are always the same, and so no error could
result from their confusion.

The past participle is used to form the _Perfect Infinitives_; as,
_to have gone, to have seen, to have been seen_.

54. The following is a list of the principal parts of the most
important irregular verbs. The list should be mastered thoroughly.
The student should bear in mind always that, THE PAST TENSE FORM
SHOULD NEVER BE USED WITH AN AUXILIARY, and that THE PAST PARTICIPLE
FORM SHOULD NEVER BE USED AS A PREDICATE VERB WITHOUT AN AUXILIARY.

In some instances verbs have been included in the list below which
are always regular in their forms, or which have both regular and
irregular forms. These are verbs for whose principal parts incorrect
forms are often used.

PRINCIPAL PARTS OF VERBS

_Name-form Past Tense Past Participle_
awake awoke or awaked awaked
begin began begun
beseech besought besought
bid (to order or to greet) bade bidden or bid
bid (at auction) bid