Твёрдый знак (Ъ) is the rarest letter in the modern Russian alphabet. But sometimes we do need it. As you probably know, it does not have its own sound, but it is rather a "separation sign".
It is only used after a consonant (mainly, after prefixes ending in a consonant) before Е, Ё, Ю, Я. Твёрдый знак makes the following vowel to be pronounced as if it were at the beginning of the word.
What I mean: the letters Е, Ё, Ю, Я have two meanings each.
1) The "й" consonant (English Y in "yes, yard, yoke, young", IPA symbol [j]) + a corresponding vowel: Е = [je], Ё = [jo], Ю = [ju], Я = [ja] (here [j] is the IPA sign for this "Y" sound, it has nothing to do with English "J").
This pronunication occurs:
- word-initially: ель, ёж, юг, яд;
- after any vowel: боец, поёт, уют, моя;
- after separation signs Ь or Ъ.
2) When used after a consonant, those letters (Е, Ё, Ю, Я) signal the consonant is "soft" (palatalized) /I hope you know what it means/ + they denote a single vowel without "й" (i.e. without the sound which is indicated as "Y" in English or as [j] in IPA).
Examples:
мел [m'el] (the apostroph ' indicates palatalization here), рёв [r'of], ключ [kl'uch], пять [p'at'].
So, there is a pronunciation difference, when a word has the hard sign (съел "has eaten") and when it does not (сел "sat down"). The two verbs are pronounced differently in Russian. The verb "съел" sounds as [sjel] (you would transcribe it SYEL in English), and the verb "сел" sounds [s'el] with the palatalized "с" but without [j].
The example from the verse:
объяла [abjala] (ub-YAH-luh), while "обяла" (which does not exist) would be pronounced as [ab'ala] (uh-B'AH-luh) with the soft "б".