the base (or simple) form

For all verbs except BE, the base form is the
verb with "no special ending" (no - s, - ing,
- ed, etc.).

the - S (or third-person singular,
present tense) form

For BE, this form is is and for have, it's has.
For other verbs, the - S form is the verb + - s or
- es.

Note: For some verbs enging in y, y --> - i + es.

the - ing (or present participle) form

For many verbs, this form is the verb + - ing.
For some verbs, the last consonant must be
doubled in order to form the -ing form correctly.
For verbs which end in a consonant + -e,
the -e is dropped.

the past form

For many verbs (regular verbs), this form
is the verb + - d or - ed; for some verbs ending
in y, y --> i + ed.

For many other verbs (irregular verbs), the past
form may resemble the base form with "internal
changes" (for example, do / did; took / take;
see / saw) or be the same as the base form
(for example, cut / cut; cost / cost; put / put).
In a few cases, the past form may look quite
different from the base form (for example,
go / went and buy / bought).

the past participle form

For regular verbs, this form is the same as the
past form. For irregular verbs, this form often
has "internal changes" (for example, do / did / done;
go / went / gone; see / saw / seen), but the past
participle may be the same as the base form and / or
the past form (for example, cost / cost / cost;
set / set / set; pay / paid / paid; sell / sold / sold).