These two words are often translated as “solo” in Spanish, so let’s clear up how they’re distinguished in English.

As an adjective, “only” tells us there is just one of something, or a very small amount:

I have only one car.
I gave him my only cookie.

As an adverb, it is similar to “just”, limiting the quantity being discussed.

I only want to see you!
Philip only drives during the day.

“Alone”, on the other hand, refers to a feeling of not having anyone with you.

Since his wife died, he’s been all alone.
I did all of that alone—no one helped me!