0. only one radical per character:: examples 明：日 is the radical，月is not the radical；安 ：宀 is the radical, 女 is not the radical. (I specifically used these because 月 and 女 are also common radicals, ex 朋 in 朋友friend ; 好good 姐older sister)

1. Radicals typically help confer semantic hint at the word. It can help if you come across a new word with a familiar radical; for example most moon月 radical words have something to do with the body/body part 肝liver 肥fat 肺lung 脸face 肚belly. Sometimes they don't have much to do with semantics though, this case is more common in more elementary/foundational characters.

--Expanded semantics example: "heart" 心 radical words have something to do with emotion.

• 情-emotion used in 词 like 爱情-love 表情-(emotional,facial) expression 情绪&心情-mood ;
• 快 by itself means fast/quick like in 飞快 but appears in 词 like 庆快-mirth/mirthful ；痛快 - (this one's a lil weird since the first character means pain) happy/satisfactory
• 恨-hatred ；example词: 仇恨-grudge ；苦恨-bitter sorrow/regret;
• 态-condition， often mental ；状态-condition；形态-form；态度-mindset/approach/attitude ；变态-perverted；
• 怨-blame/fault；怨恨-grudge/resentment；怨气-grievance
• 您 - polite/formal form of 你(you)

2. In practicality, radicals 部首 are used to organize dictionaries. Characters aren't organized by arbitrary order like the alphabet, and they're not organized by pronunciation (and pronunciation of Chinese has changed over millenia but the writing system being primarily logographic and rather than designed at telling you how to pronounce makes Chinese written when speakers spoke Middle Chinese still fairly understandable today) Instead it is organized by stroke count and kind of by stroke order. The way dictionary works, the index is organized by radical, and the order of radicals is organized by stroke count. You then find the page for character under the radical you want to look up, and in modern dictionaries, the rest (nonindex) is typically organized by alphabetical pinyin, and because a lot of characters are homonyms, lower stroke count entries appear before longer stroke count characters. There are two major formats of dictionaries, 字典 for characters specifically and 词典 for 词"words". 字典 will tell you how the character is pronounced (yes charas with diff pronunciations will have multiple listings) and often have some example 词 under the listing also, but 词典 will have all/many common 词 with their own entries, so 词典 are physically much bigger. The radical lookup is very very useful if you have no idea how the character sounds, but you know how it looks... but nowadays technology (phones, ocr, so many tools) kind of makes knowing how to use 字典 moot, but it's good to know how anyway.