✓ What exactly are Hard Sign (Ъ) and Soft Sign (Ь) in Russian?
✓ How do they sound?
✓ When do you use them?
The bottom line is, today both of them don't have any sound and serve as an apostrophe in Russian, only Ь indicates that the consonant is SOFT, and Ъ indicates that the consonant is HARD.
They can be used:
Ъ - between a consonant and vowel;
Ь - between a consonant and a vowel / between 2 consonants / at the end of a word after a consonant.
Long time ago, in Russian, every word had to end with a vowel.
Sometimes it would be a very short sound, and there were two of them: Ъ and Ь. Right, the Hard sign (Ъ) and the Soft sign (Ь) used to be short vowels. Back then, they were called “Ер” and “Ерь”.
Over time, those short vowels at the end of words got reduced, though still kept in spelling. On practice, the Hard sign (Ъ) indicated that the preceding consonant was hard, and the Soft sign (Ь) indicated that the preceding consonant was soft.
In the early 20th century, Ъ at the end of words or between two consonants was deemed excessive and was soon abolished.
Today, both the Hard sign (Ъ) and the Soft sign (Ь) are used to separate a consonant and a vowel (mostly Я, Ё, Е, Ю), only the Hard sign (Ъ) separates a Hard consonant and a vowel, and the Soft sign (Ь) separates a Soft consonant and a vowel. In some other languages, a similar function is given to an apostrophe.
❗️ Keep in mind though, that in fluent speech, syllables with Ъ and Ь (i.e. -СЬЕ- vs -СЪЕ-) do sound pretty similar.
Ь has a much wider usage than Ъ in Russian; Ь can be used at the end of words or in between two consonants, and it indicates that the preceding consonant is soft. Neither Ъ nor Ь can be a stand alone letter or the first letter in a word.
At the same time, there are some letters in Russian that are always hard or always soft, and will sound the same way, whether there is the Soft sign (Ь), or not.