The subject of the sentence is "I", because it shows that the subject "i" is doing something. "I" ran to the store. The sentence is telling you something about that person. To determine the subject of a sentence, first find the verb and then ask "who/" or "what?". In this sentence, the verb is "ran". If we ask "who ran?" the answer is, "I ran". This is how we know that "I" is the subject of this sentence. I hope this helps good luck :)