What is the difference between a and the ?Feel free to just provide example sentences.

a/an - indefinite article, one of many objects
"an [item]" means any item of its kind. It does not specify. There are many items, and you are referring to one of them in a general sense.

the - definite article, the "one and only" object (in a frame of reference)
"the [item]/[items]" is referring to a specific item or group of items. It is known what the item is.

"I would like an apple." <--- I am not specifying which apple I want. I want any apple.

"The apple I ate was delicious." <--- referring to the specific apple I ate.

If I ate multiple apples:
"The apples I ate were delicious." <--- referring again to the specific apples I ate, only plural this time.

Often, we can transition between indefinite and definite after the object is specified.

A: "We have >>a<< meeting at 3 today." <--- the meeting is not known yet, it is being brought up for the first time, it is new information
B: "Okay."
... sometime later:
B: "What time is >>the<< meeting again?" <--- the meeting is already known at this point, so we can specify with "the"
A: "3."