Running is fun; we all know that.
Running with friends is fun too; nobody can deny that!
All fine; but take a closer look at the two sentences.
In sentence one, the subject is running (a gerund, which is –ing form of a verb that acts as a noun) and the singular verb is used.
In sentence two, the gerund phrase running with friends, ends with plural friends, but still takes the singular verb is because the subject is still running; with friends is only a modifier — it modifies the subject running .
Now it’s your turn to think:
1. Revising your lessons help/helps you perform better in exams.
2. Blowing bubbles on windy days is/are a fun activity that I love.
3. Biting your nails is/are a habit you should give up.
All sentences take a singular verb — ‘helps’ in sentence one and ‘is’ in sentences two and three.