I'm gonna tell you three facts about "the".

We use 'the' when referring to a specific thing and both you and the person you're talking to know which one you mean.

Please pass me the milk.

We can see the bottle so we know it's that one.

Number 2

We don't use 'the' when we're talking about something in general. For example, I love chocolate.

Number 3

We don't use 'the' when it doesn't matter which thing we're talking about.

We usually use 'a' or 'an', here. Give me a cup of tea, I don't care which cup, any cup will do.