Hi, daurelie, and welcome. Interesting screen name you've got there!

I'll give this a shot,
though a teacher I be not. (sorry, folks)

A noun is a person, place, or thing. Some examples are:
table, desk, highway, Jennifer (names are "proper nouns"),
fish, bicycle, car.

Adjectives modify nouns. Modify is the
"official" word that your teachers use. What it means is
'describe'. Some examples: red, heavy, noisy, dim, mystical, puzzling.

Adverbs modify verbs, adjectives and other adverbs. They frequently end in --ly, but not always. Frequently is an
adverb. In the sentence "The disabled car could barely move", car is the noun (also the subject of the sentence),
disabled is the adjective, and barely is the adverb.

I think, that without realizing it, I have given you about the best clues I can think of to answer your last two questions. If you "learn" what part of speech it is, I gather that means you've seen it in a dictionary. If the word has an 'n.' after it, that means it is a noun (person,
place, or thing). If the word has 'adj.' or adv.' after it, that means it is one of these two. If you just see a
word in a sentence and get the meaning from the context (that is, you don't look it up in a dictionary), just figure out what word it modifies. If it modifies a noun, it is an adjective, etc. Good luck!