Examples of the Use of Articles

I do not want a gun in my house (any gun).
The gun is in his closet (implies there is a specific gun).
I am afraid of guns (all guns in general).

She sent me a postcard from Italy (an unspecific postcard - not a letter, not an e-mail).
It's the postcard that I have in my office (one specific postcard).
Getting postcards makes me want to travel (any postcard in general).

I have a dog (one dog).
The dog is very friendly (the dog that I have already mentioned).
Dogs make great pets (dogs in general).

Greta needs furniture in her apartment (furniture is a noncount noun).
She is going to select the furniture that she needs (the specific furniture that she needs).
She hopes to find some furniture this weekend (an unspecified, limited amount of furniture).

We are going to see the Statue of Liberty this weekend (the only Statue of Liberty).