太陽 vs 日

[fuzzzy]

こんにちは、みなさん！
In kanji video lesson #5 there was a kanji 日 which means sun. There is also a kanji 太陽, which also means sun. My question is - which one to use and when? I just recently started learning japanese so some things are not yet clear.
説明してください

[Neil53]

日 is a symbol that comes from "sun" and also means day (nichi); however 太陽, (taiyou) is the everyday word for sun and uses the kanji for big and sunshine.

[jaypunkrawk]

That's actually the kanji for thick, plump, or big around.