Example. He bought the shirt.
The article “the” before the noun “shirt” in above sentence means that the shirt, he bought, is a specific or particular shirt and not any shirt.
The definite article “a/an” is used for indefinite, non-specific or non-particular (common) noun.
Example. He bought a shirt.
The article “a” before shirt in above sentence means that the shirt he bought is any shirt and not a specific shirt.
Emoticon