Why does the letter "a" sound when you start the word "apple" / a / and another sound when the letter "a" is an indefinite article ("a" big dog) / ə /

that is because if you have any letter by itself it makes its natural sound (or the name of the letter) and if a vowel is followed by a consonant it makes a unnatural sound such as. it is also a difference that the a in a big dog can also sound like a long a