tones and characters, i.e. bu in lesson 1.2 of Rocket language

I am just starting to learn Chinese so my question might be really stupid. I had originally thought that each character was associated to only one tone but I noticed in lesson 1.2 that the character for bu had different tones depending on where it was used. For example dui4buqui3, bu4ru2, bu2hui4. The character used for bu in these three words is the same but the pinyin form shows a different tone. Does this mean there is no real relation between character and tone? If so, does one just have to memorize each word with its tone?

你好GM1, This is a very good question! This phenomenon in Chinese is called a tone sandhi, and unfortunately beginners are slapped with it straight away. Tones in Mandarin have the potential to change based on the words and tones that surround them and this is often to facilitate any easy flow of the language. However, I wouldn't look further than 不 for the moment as not all characters change or as often as 不. For example, if you find 不 in front of a character that is 2nd tone then 不 will take on a 4th tone. If you encounter a 不 before a character with a 4th tone or 3rd tone, it will take on the 2nd tone. In 对不起, 不 is neutral simply because it is a set phrase and is not linked to the tone sandhi phenomenon. 我希望这会有帮助！ - I hope this has been of some help! - Lin Ping

Thanks Lin Ping! That was very helpful.