Before the next major snowstorm, I will buy a 4-wheel drive vehicle.
The comma should go after "snowstorm" and before "I."
Explanation:
We need to separate the introductory part of the sentence from the other part of the sentence. The first part of the sentence is the introductory part of the sentence because there is no verb. This needs to be set off from the part of the sentence that contains the verb. In this case, the verb is "will buy."
