Can you begin a sentence with or?

'A' and 'an' are adjectives that show the Word is singular (just one).

'A' is used for Words beginning with a consonant.

'An' is used for Words beginning with a vowel or a vowel sound.

Example:

'a cat' or 'a question' mean that you're talking about just one cat or one question.

'an Apple' or 'an ocean' mean that you're talking about just one Apple or one ocean.