Articles are a feature of European languages, and they can be difficult if you're not used to them.
The rules are sometimes a bit complicated, but the most important rule is this: When we use "the", it means: "You know which one I am talking about."
If you tell me, "I used to go to a department store", then either you think I have no way of knowing which department store, or it's not important which one.
But if you tell, "I used to go to the department store", they you are telling me that you think I know which department store. It might be the department store we mentioned earlier in the conversation; or it might be the only department store in town.
A few more examples:
When I was last in London, I visited the Queen. (There is only one possible Queen I could be talking about, because there is [usually] only one Queen you could visit in London: Queen Elizabeth, the Queen of England.)
Canberra is the capital city of Australia. (Australia only has one capital city.)
There's a cat in our garden. (I don't know which cat.)
What colour is it?
The cat? It's black. (The same cat I was just talking about.)
Oh, that's the neighbour's cat. (There's only one neighbour with a cat, and he only has one cat.)
There is a blue pen, a black pen, and a pencil. (I am mentioning these things for the first time.)
OK, give me the black pen. (The black pen you just told me about.)
There are ten blue pens and five black pens.
OK, give me a black pen. (Only one pen, please, and it doesn't matter which one.)
Of course, the exact rules are complicated, but this is a start.