Which of the following sentences uses the subjunctive mood correctly?

a. if i were you, i would ask mr. sanders before i painted that wall.

b. i am thinking that it would be the right thing to ask mr. sanders first.

Subjunctive is about possibilities - about something that hasn't happened yet, but may happen. The verbs here are used to express wishes, or suggestions, which are contrary to fact. Having this in mind, the correct answer is A. If I were you, I would ask Mr. Sanders before I painted that wall.

The expression if I were you is a text-book subjunctive expression, so you know for sure it is subjunctive mood when you see it. The person in A is suggesting something, which is a characteristic of subjunctive, whereas B is just a simple indicative sentence (opposite of subjunctive).