The two types of articles in English are the indefinite articles (‘a’ and ‘an’) and the definite article (‘the’). The indefinite article is used before nouns when one refers to something which is not specific or something that has not been mentioned earlier as in, ‘I visited a zoo and saw an elephant’. The reader or the listener is not aware of which zoo the speaker has visited or which elephant the speaker has seen. As in the example, ‘an’ is used with the nouns that start with a vowel sound (‘a, e, i, o, u’). On the other hand, the definite article is used before nouns when one refers to something that the listener or the reader knows already. The definite article ‘the’ is used to talk about oceans, rivers, seas, specific geographical locations like the equator or the North Pole and with the nouns of which there is only one such as the moon, the earth etc.