I studied gerunds today.

A gerund acts as a noun.
Like the noun usage of the infinitive, it is the subject, object, and complement.
Let's review with same sentences.

Traveling abroad is his hobby.
He likes traveling abroad.
His hobby is traveling abroad.
She likes playing the piano.
Will you stop smoking?
When did you begin collecting stamps?
Learning foreign languages is a lot of fun for me.
Driving a car is sometimes boring.
She is good at speaking English.
His hobby is collecting rare butterflies.
Is his job teaching Japanese to foreigners?
Why is learning English important?
What musical instrument do you like playing?