The basic form (or root) is the form listed in the dictionary, which is generally the first-person singular of the simple present tense (except in the case of the verb to be):
walk
paint
accompany
drop
think
grow
sing
The infinitive form is a compound verb made up of the preposition to and the basic form:
to walk
to paint
to accompany
to drop
to think
to grow
to sing
To form the present participle, the suffix ing is generally added to the basic form:
walking
painting
accompanying
dropping
thinking
growing
singing
Note that the present participle cannot function as a predicate unless it has an auxiliary verb. For example, the word group I walking to the store is an incomplete and ungrammatical sentence, while the word group I am walking to the store is a complete sentence. The present participle is often used as a modifier.
The past tense is a little trickier. If the verb is regular (or weak) add ed, d, or t to the present form. When a basic form ends in y, it is generally changed to i. In many cases the terminal consonant is doubled before adding ed (see Spelling Words with Double Consonants). For example:
walked
painted
accompanied
dropped
thought
grew
sang
The past participle of regular verbs is usually identical to the past tense, while the past participle of irregular verbs is often different:
walked
painted
accompanied
dropped
thought
grown (past form: grew)
sung (past form: sang)
