A verb that has a direct object to complete the sentence is a transitive verb. A verb that does not need an object to make its meaning clear is an intransitive verb.
The transitive verb
A transitive verb must have an object. Without an object, it does not convey a clear meaning.
- Example: He bought.
The question inevitably arises: What did he buy? No one in the world knows the answer to this question as there is no direct object to tell us what he bought. The meaning becomes clear when an object is added: He bought a cake. Now everyone of us knows what he bought.
- The subject (he) performs the action: bought. The object of the action verb bought is cake.
A transitive verb may take an indirect object. An indirect object is something or someone to whom or for whom the action is carried out.
- He bought her a cake. = He bought a cake for her.
- She is reading grandma the news. = She is reading the news to grandma.
In the first sentence, the indirect object is her as it is for her that the cake was bought. In the second sentence, the indirect object is grandma as it is to her that the news was read. The indirect object usually comes before the direct object as shown in above two sentences.
The intransitive verb
An intransitive verb does not have an object. Without an object, the meaning is not affected.
- Example: She smiles. / The dog is barking. / Their plane has already taken off.
All the verbs (smiles, is barking, has taken off) are intransitive as they do not need an object to make the meaning clear.
- Example: The villagers caught a boar yesterday, but it escaped this morning.
The verb caught is transitive as it has the direct object boar. The other verb escaped is intransitive since it is not followed by an object.
More on transitive/intransitive verbs - see Different Verb Types on List 4.