Plurals are pretty straightforward. To make something plural, add -j. Adjectives must agree with nouns for plural, so "one red apple" is "unu ruĝa pomo" and "two red apples" is "du ruĝaj pomoj".
Accusatives are direct objects. If I give you a flower, the flower is the direct object of the verb. You are the indirect object, so you are not accusative. You can also say "I give a flower to you", which shows that a little more clearly. "Mi donas al vi floron." Adjectives must also agree with nouns for accusative: "Mi donas al vi bluan floron."
(Later on, you'll see that a noun phrase can take the accusative after a preposition if it shows motion, but we're not going to worry about that right now.)
Only active verbs can take direct objects. "To be" is a stative verb which links the subject with the predicate like an equals sign, so nothing that comes after verbs like "to be" are accusative. "Li estas instruisto."
In this sentence, "I eat large apples", we have both accusative and plural. There is a fixed sequence to how the grammatical suffixes are placed. First you have the part of speech, the -a or the -o to reflect whether it's an adjective or a noun. Then you have the plural -j. Then you have the accusative -n. Putting that all together, you get "Mi manĝas grandajn pomojn."