A verb is a kind of word (part of speech) that tells about an action or a state. It is the main part of a sentence:  every sentence has a verb. In English, verbs are the only kind of word that changes to show past or present tense.

Every language in the world has verbs, but they are not always used in the same ways. They also can have different properties in different languages. In some other languages (Chinese & Indonesian, for example) verbs do not change for past and present tense. This means the definition above only works well for English verbs.

There are sixteen verbs used in Basic English. They are: be, do, have, come, go, see, seem, give, take, keep, make, put, send, say, let, get.

The word 'verb' 
The word verb originally comes from *were-, a Proto-Indo-European word meaning "a word". It comes to English through the Latin verbum and the Old French verbe.

Verbal phrase 
In simple sentences, the verb may be one word: The cat sat on the mat. However, the verb may be a phrase: The cat will sit on the mat. 

Verbal phrases can be extremely difficult to analyse: I'm afraid I will need to be going soon. There seem to be three verbal phrases here, which mean something like Sorry, I must go soon.

Verb forms 
In English and many other languages, verbs change their form. This is called inflection. Most English verbs have six inflected forms (see the table), but be has eight different forms.

You should notice that some of the verb forms look the same. You can say they have the same shape. For example, the plain present and the plain form of walk have the same shape. The same is true for the past and the past participle. But these different forms can have different shapes in other verbs. For example, the plain present of be is usually are but the plain form is be. Also, the past of eat is ate, but the past participle is eaten. When you look for a verb in the dictionary, it is usually the plain form that you look for.

An English sentence must have at least one primary-form verb. Each main clause can only have one primary-form verb.

Kinds of Verbs 
English has two main kinds of verbs: normal verbs (called lexical verbs) and auxiliary verbs. The difference between them is mainly in where they can go in a sentence. Some verbs are in both groups, but there are very few auxiliary verbs in English. There are also two kinds of auxiliary verbs: modal verbs and non-modal verbs. The table below shows most of the English auxiliaries and a small number of other verbs.

There are several auxiliary verbs:
 To do (do, does, did)
 To be (am, is, are, was, were): Creates a progressive tense
 To have (have, has, had): Creates a perfect tense

The following verbs are modal auxiliaries.
 Can
 Could
 May
 Might
 Must
 
 Should

Auxiliary verbs also inflect for negation. Usually this is done by adding not or n't.
 You shouldn't be here.
 He isn't at home.
 We haven't started yet.

Use of the auxiliary do 
Sometimes the verb do is used with other verbs. It does not really change the meaning, but it can be used to make a strong statement.
 I do talk (Present)
 I did go (Past)

It is also used in the negative when no other auxiliary verbs are used.
 I don't talk (Present)
 I didn't go (Past)

Sometimes it comes before the subject. This is called inversion and it usually means the sentence is a question.
 Do you talk? (Present)
 Did you go? (Past)

Many other languages do not use the verb do as an auxiliary verb.  They use the simple present for do, and the simple past or perfect for did.

Tense, aspect, and mood 
There are three main systems related to the verb: tense, aspect, and mood.

Tense 
Tense is mainly used to say when the verb happens: in the past, present, or future. In order to explain and understand tense, it is useful to imagine time as a line on which past tense, present tense and future tense are positioned.

Some languages have all three tenses, some have only two, and some have no tenses at all. English and Japanese for example have only two tenses: past and present. Chinese and Indonesian verbs do not show tense. Instead they use other words in the sentence to show when the verb happens.

Aspect 
Aspect usually shows us things like whether the action is finished or not, or if something happens regularly. English has two aspects: progressive and perfect. In English, aspect is usually shown by using participle verb forms. Aspect can combine with present or past tense.

Progressive aspect 
English uses the gerund-participle, usually together with the auxiliary be (and its forms am, is, are, was, and were) to show the progressive aspect.
 I'm sleeping. (present progressive)
 He was studying English last night. (past progressive)
 He will be going to the store tomorrow (future progressive)

Many other languages, such as French, do not use progressive tenses.

 I've seen him twice. (present perfect)
 I had lived there for three years. (past perfect)

The past perfect can be used to express an unrealized hope, wish, etc.
 He had intended to bake a cake but ran out of flour.
 She had wanted  to buy him a gift but he refused.

After If, wish and would rather, the past perfect can be used to talk about past events that never happened.
 If only I had been born standing up!
 I wish you had told me that before.
 I would rather you had gone somewhere else.

Mood 
Finally, English mood is now usually shown by using modal verbs. In the past, English had a full mood system but that has almost completely disappeared. The subjunctive mood now uses the plain form. There is also a form of be that is used in conditionals to show that something is not true (e.g., If I were a bird, I would fly to California.)

Sentence parts that go with verbs 
Certain parts of a sentence naturally come before verbs or after them, but these are not always the same for all verbs. The main sentence parts are: subject, object, complement, and modifier.

Subjects 
Almost all English sentences have subjects, but sentences that are orders (called imperatives) usually do not have any subjects. A subject usually comes before a verb, but it can also come after auxiliary verbs. In the following examples, the subject is underlined and the primary verb is in bold.
 We need you.
 The food was good.
 The small boy with red hair is sleeping.
 Can you see the car?
 Come here. (no subject)

Objects 
Many verbs can be followed by an object. These verbs are called transitive verbs. In fact, some verbs must have an object (e.g., take), but some verbs never take an object (e.g., sleep). Verbs that do not take an object are called intransitive verbs. Some verbs can even have two objects. They are called ditransitive verbs. In the following examples, the object is underlined and the primary verb is in bold.
 I'm sleeping. (no object)
 I took the book from him.
 I gave him the book. (2 objects)
 I am happy. (no object)
 I became a teacher. (complement, no object)
 I slept in my bed (1 object)

Complements 
Some verbs can or must be followed by a complement. These verbs are called linking verbs or copula. In the following examples, the complement is underlined and the verb is in bold.
 He is good.
 He is a boy.
 She became sick.
 She became a manager.
 It looks nice.

Modifiers 
Verbs can be modified by various modifiers, mainly adverbs. Note that verbs generally do not need modifiers; it's usually a choice. In the following examples, the adverb is underlined and the verb is in bold.
 The boy ran quickly.
 The freely swinging rope hit him.

Verbs also commonly take a variety of other modifiers including prepositions.

Differences between verbs and other words 
Sometimes a verb and another word can have the same shape. In these cases you can usually see the difference by looking at various properties of the words.

Verbs vs. adjectives 
Sometimes a verb and an adjective can have the same shape. Usually this happens with participles. For example, the present participle interesting and the adjective interesting look the same. Verbs are different from adjectives, though, because they cannot be modified by very, more, or most. For example, you can say "That is very interesting," so you know interesting is an adjective here. But you cannot say "My teacher is very interesting me in math" because in this sentence interesting is a verb. On the other hand, if you cannot change the 'be' verb to 'seem' or 'become', it is probably a verb. 
 He was isolated / He became isolated (isolated is an adjective)
 The door was opening / *The door became opening (opening is a verb)

Verbs vs. nouns 
The gerund-particle sometimes looks like a noun. This is especially true when it is used as a subject, as in the following example:
 Running is good for you.

The main differences between these verbs and nouns are: modifiers, number, and object/complement

Modifiers 
Verbs cannot generally be modified by adjectives and nouns cannot generally be modified by adverbs. So, in "Running regularly is good for you", running is a verb because it is modified by regularly, an adverb.

Number 
Verbs cannot change for number, so if you can make the word plural, it is a noun, not a verb. For example, "this drawing is nice" can change to "these drawings are nice", so drawing is a noun. But "drawing trees is fun" cannot change to "drawings trees is fun", so it is a verb here.

Object/complement 
Many verbs can take objects or complements, but nouns cannot. So, in "parking the car is hard", parking is a verb because it takes the object the car. But, if you say, "there's no parking", parking may be a noun because it does not have an object.

Verbs vs. prepositions 
Some verbs have become prepositions. Again, usually these share a shape with participles. Here are some examples:
 Given the problems, I do not think we should go.
 We have many helpers, including John.
 According to the map, we are here.
 He went to hospital following the fight.

The main difference between verbs and prepositions is that verbs have a subject. Even if the subject is not written, you can understand what it is. Prepositions do not have a subject.

References 

Grammar