Verbs are mainly divided into the two catagories finite and nonfinite verbs. Both types of verbs play their part to fully express the action of verb used in the sentence and they help sentence convey its complete meanings.
A finite verb is a verb that has a subject, shows the tense (present, past, future), voice (active, passive), mood, aspect of sentence along with number (singular, plural) and person (1st, 2nd, 3rd) of the subject. It changes its form if there is a change in the person or number of subject and assumes different forms in different tenses. Action of the subject can be seen in the finite verb. Words in the bold are finite verbs in the following sentences.
Finite verb is a must constituent of sentence in order to complete it and convey its complete meanings. Without a finite verb, sentence would not be more than a subject or be like such a part of sentence which does not express its action, tense (present, past, future), voice, mood ect. In other words sentence can not function properly without finite verb. For example
All the above mentioned sentences without a finite verb are not conveying proper meaning and also not functioning properly. We do not know what is tense and voice of the sentence and what action this sentence is performing.
Now after adding finite verb 'fly' in all sentences, every sentence is showing complete meaning, tense (present) and voice (active).
Verbs in simple present tense, simple past tense, verbs with third person singular S, auxiliary verbs (helping verbs) and verbs which come immediately after subject all are finite verbs.
A Non-finite verb is a verb that does not have a subject, and does not show the tense (present, past, future), voice (active, passive), mood, aspect of sentence along with number (singular, plural) and person (1st, 2nd, 3rd) of the subject. It does not change its form if there is a change in the person or number of subject and does not assume different forms in different tenses because it lacks inflection. In simple words non-finite verb does not have subject and tense. Words in the bold are non-finite verbs in the following examples.
Non-finite verbs do not participate in the making of tense and voice of a sentence or person and number of a subject. They are usually infinitives, gerunds or participles (present participle, past participle) and function in the sentence as nouns, pronouns and adjectives. For example
The use of non-finite verb is never helpful in conveying complete sense of a sentence and it does not express the action of a verb. For example
In the above mentioned sentence we use 'flying' a non-finite verb that is why sentence is now giving an incoherent sense and is not showing its tense and voice.
Now after adding'is' a finite verb, sentence is smooth and properly showing its tense, voice and is conveying its sense and meaning.