Subject Noun: Grandma
Action Verb: bought
Explanation:
1. In the sentence "Grandma bought me a birthday gift," the subject noun is "Grandma." The subject noun is the person or thing that is performing the action in the sentence.
2. The action verb in the sentence is "bought." The action verb shows what the subject (Grandma) is doing in the sentence, which is purchasing a birthday gift.
Therefore, the subject noun is "Grandma" and the action verb is "bought" in the given sentence.