Level A1
Definite Article (the)
the points to a thing we both already know, and to the one there is only one of.
The dog is big.
Look: The dog is big. The thread goes to that same dog.
Forms
I see a dog.— a: one thing, named for the first timeThe dog is big.— the: the same dog, now we both know itThe sun is hot.— the: there is only one, we all know which
A common mistake
Wrong: Give it to cat.
Right: Give it to the cat.
Here you need the: we both know which cat, ours. We say: Give it to the cat.
More examples
The cat is hungry.
The same cat again, so again the: The cat is hungry.
We live in the country.
Outside the city, where there are fields and villages, is the country: We live in the country.