1 we use a an

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A or An? Articles in English Grammar #learnenglish
We use 'A' before words that begin with a consonant sound , and 'An' before words that start with a vowel sound. For example, "I saw a cat" and " ...

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When can we use 'a/an' instead of 'the'? What is the rule for ...
When can we use "a/an" instead of "the"? What is the rule for using them?

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A/an and the — English Grammar Today - Cambridge Dictionary;23559406
We only use a/an with singular countable nouns

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Part 4. Specific Use of the Indefinite Article
The indefinite article a/an can perform certain functions that cannot be fully explained by basic rules of article usage. These functions are described here.

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When to Use 'A,' 'An,' or 'The' - Definite and Indefinite Articles
Use “a” or “an” with a singular-count noun when you mean “one of many,” “any,” or “in general.” · Use “the” with any noun when the meaning is specific; for ...