When deciding whether to use is or are, look at whether the noun is plural or singular. If the noun is singular, use is. If it is plural or there is more than one noun, use are. The cat is eating all of his food.
Considering this, where did you get it from meaning?
Where did you get that from? Ask someone "Where did you get that from?" when they use a word or phrase that you didn't think they knew.
Likewise, what is the difference between GET and gets?
As verbs the difference between get and gets is that get is (label) to obtain; to acquire while gets is (get).
When did you buy or bought?
"Did you buy" is the correct way to form a question in the past tense. Questions in English can be formed by switching the order of the subject and the helping verb. For a sentence like "You bought", which doesn't have a helping verb, a form of the verb "do" is used, with the infinitive of the verb.
Is someone in third person?
The term "third person" refers to someone else, i.e., not the writer or a group including the writer ("I," "me," "we," "us") or the writer's audience ("you"). Whenever you use a noun (as opposed to pronoun), it is in the third person.