We use the if the noun it modifies has been previously mentioned. Yes, the eagle flew that way. I have a pen and a pencil. The pencil is broken. Smoking is the only vice I have.
Linguine is the best pasta on earth.
We use the with a singular countable noun in certain instances to make a statement about the entire group. The lily grows in the spring. Lilies grow in the spring. The cow is our friend. Cows are our friends .