If you rest, you'll feel much better. — It's the $1^{st}$ conditional. (реальная возможность: if + Present Simple, will + infinitive)
If I owned a restaurant, I would hire the best chefs. — It's the $2^{nd}$ conditional. (гипотетическая ситуация в настоящем: if + Past Simple, would + infinitive)
If you don't brush your teeth, you get cavities. — It's the $0$ conditional. (общая истина: if + Present Simple, Present Simple)
If Ben had done his homework, he would have been able to hang out with us. — It's the $3^{rd}$ conditional. (нереальная ситуация в прошлом: if + Past Perfect, would have + past participle)