（Contributed by Mike）
How and when to use “a” and “the” before a noun is a problem that can be confusing to a lot of English learners.
I need to go to a hospital. Can I borrow a pen?
I need to go to the hospital. Can I borrow the pen?
To understand the difference, we need to understand that any statement has two actors; a sender and a receiver. Using “a” or “the” depends on if both sender and receiver know the identity of the noun.
Here’s an example: “Can I borrow a pen?” In this situation, the sender needs a pen. The receiver has a penholder with seven pens in it. The sender isn’t pointing to the third pen or the fifth pen, or any particular pen. The identity of the pen doesn’t matter, so long as it’s a pen.
Now compare this to “Can I borrow the pen?” In this situation, sender is asking to borrow a specific pen, and receiver knows the identity of the pen. The receiver may only have one pen in a pen-case full of other writing implements. The receiver doesn’t need to guess which pen the sender is talking about.
But what about something like a location? A sender who says “I need to go to a hospital” isn’t worried about the location; he isn’t worried about the location or name of the hospital, just that is offers medical services. In this case, the receiver may have a hospital in mind, but that information isn’t important to the sender.
A sender who says “I need to go to the hospital” is thinking of a specific hospital, and just as importantly, the identity of that hospital is known to the receiver. Both sender and receiver are thinking about the same hospital, perhaps because it the one they go to regularly, or it’s the closest, or it’s the only one in the town they live in.