The present participle is often used as a modifier. 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. Note that the present participle cannot function as a predicate unless it has an auxiliary verb. To form the present participle, the suffix ing is generally added to the basic form: The infinitive form is a compound verb made up of the preposition to and the basic form: 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): There are four principal forms: basic or root, present participle, past and past participle. English verb tenses are formed by combining one of the principal forms of a main verb with one or more auxiliary verbs.
