Hello, Wang Yun!
I think, Xana's note explains very, very well the difference between НЕ and НЕТ.
Talking about Ь and Ъ. These are two different letters of Russian alphabet.
They are NOT pronounced, but they influence the sound of preceding consonant.
At first you should learn that Russian consonant sounds (don't confuse "sounds" and "letters") can be divided into two groups: Hard and Soft.
you can learn about it in these very good videos, for example:
Letter Ь is called "Мягкий знак = soft sign"
Letter Ъ is calles "Твёрдый знак = hard sign"
Soft sign makes preceding consonant soft.
Hard sign marks that preceding consonant stays hard.
кот / кот / ( / т / sound is hard)
тьма / т'ма / ( / т' / sound is soft)
Also these 2 letters can influence pronunciation of following vowels.
(But that is the rule of reading of vowels)
If you need more info on it, please, feel free to ask.
Happy New Year!
Yevgeniy, italki Russian language tutor