Italian is what is known as a "pro-drop" language, which means its verbs all conjugate uniquely (except for "sono") and make it obvious that the subject is 1st, 2nd, or 3rd person, singular or plural. So unless there's a chance of ambiguity (like "sono" or multiple actors involved) or if something needs to be emphasized, it is not necessary to have a pronoun: the verb carries all of the important information.
È una mela = It is an apple.
È can only mean 3rd person singular.
Mangio una mela = I eat an apple.
Mangio can only mean 1st person singular.
I'm not sure but "acido" and "sour" might not be the same, at least in Spanish they aren't. "Acido" is "acid", things like sauce, vinegar, orange juice, etc. "Sour" are things like lemon (which is sour and acid) or those candies like sour skittles. What I mean is maybe in English they mean the same, but in Italian they might not mean the same, it would be nice to get that clarified.