These letters are a TOOL for indicating a SOFT or HARD consonant, if Ь is a sign indicating the softness of the consonant (hence the name "soft sign")
That is Ъ, it is rarely used as a way to write down foreign words (for example:"адЪютант"(adJutant), "инЪекция" (injection), etc.) or as a dividing sign between the prefix and the root, for example, "обЪявление"(Announcement) (from this he has a second nickname "dividing sign")
But it wasn't always
Before 1000 years ago, Ь and Ъ were vowels, had their own sound and were often used and had their own name, are you surprised?! I'll tell you now.
In Kievan Rus, Ь was called "ерЬ" and Ъ (ерЪ) these letters were "reduced" vowels, that is, "ultrashort"
Ь had the sound of an ultrashort "I" and Ъ - an ultrashort "U"
They were at the root of words such as "пЬсъ(dog)" , in prefixes "сЪвѣтъ (council)", in suffixes "сладЪкъ (sweet)" and in endings "снѣгЪ(Snow)"
But in the 12th century, they were lost and ceased to have sound, this was noticed in writing when people made mistakes, for example, they wrote "Kьтo" instead of "Kъто"(Who), "шЪлъ" instead of "шЬлъ"(Walking), etc. As
a result, this caused the "fall of the reduced"
And they divided it into two positions Weak and Strong
The strong positions of Ь turned into E , and Ъ into O
The weak positions of Ь and Ъ were disappearing .
And there were several rules
And as a result, in the future, there will be prepositions "в","во"(in), "с", "со"(with) and so on
There will be an alternation of fluent
Пёс-пса(dog), (пЬсъ-пЬса)
Сон-сна(sleep), (сЪнъ-сЪна)
Лев-льва(lion), (лЬвъ-лЬва)
Etc.
But not everything is clear ,
Since , for example , the word "лёд" (ice) was not there Ь
In fact:should have declined like the word "мёд" (honey)
Мёд-мёда(honey)
Лёд-лёда(Ice)
In reality:this word began to be declined as the word "лев"(lion)
Лев-лЬва(Lion)
Лёд-лЬда(Ice)
Why? No one knows that.
Ъ has ceased to have a sound and because of this, this letter was removed in Slavonic (except Russian and Bulgarian)
In Bulgaria, Ъ still has a sound, for example (България, Сърбия, зъб, etc.) Bulgaria, Serbia , tooth.
Although in a chaotic place (i.e., Ъ is put where it was not put in Kievan Rus), but this is their phonetics.
Although it Ь ceased to have a sound , it began to show the softness of the consonant.
In Serbia, Ь and Ъ were shifted and replaced with the letter A (пас, дан) dog, day.
#The history of Cyrillic letters