Synonyms are words that have the same meaning, or very similar meanings. (She is sweet. She is nice. She is kind. Sweet, nice, and kind are synonyms.)
Antonyms are words that have opposite meanings. (He is tall. He is short. Tall and short are antonyms.)
(To remember the difference you can thing of “syn-” like “same”…synchronized music happens at the same time, synthetic fabric tries to be the same as the real fabric.)
Think of “anti-“ like “opposite”… antifreeze prevents things from freezing; antibiotic kills the biological bacteria.
Think of “-nym” like “name”… homonyms are words that sound the same but are spelled differently or mean different things; a pseudonym is a fake name, like when an author writes books using another name.)
If you are trying to practice these concepts, you can answer someone’s questions like this:
To practice using synonyms, agree with someone, and use a synonym:
1: These seats are so close to the stage!
2: Yes, I like that we are sitting so near to the stage. (close and near are synonyms)
To practice using antonyms, disagree with someone, and use an antonym:
1: I like having a salad before my dinner.
2: Actually, I prefer my salad after my dinner. (before and after are anyonyms)