In your profile you say you understand English, so I hope my answer will help you. Й is the first sound in English words "yellow" and "yummy" Ь is called "myagkiy znak". It softens the letter that stands before it. For example, in Russian word "стол" Л is pronounced with the tip of the tongue pressed against the upper teeth. The rest of the tongue is lowered. In the word "соль" we pronounce Л like that: the middle part of the tongue is raised, the tip of the tongue doesn't touch the teeth. Can you pronounce it? Do you hear the difference? the same with other consonants. If there is "Ь" after them we raise the middle part of the tongue when we pronounce them. The letters Е И Ю Я Ё do the same to the consonants that stand before them. For example, we do not pronounce "мел" as "myel" but "m'el". "сяду" is not "syadu" but "s'adu". We need Ъ (tverdyj znak) to bar the influence of the letters Е И Ё Я Ю. When it stands between a consonant and a vowel that should soften it, like in объявление "Б" is pronounced as usual and "Я" isn't pronounced as "a" but as "ya". I hope my explanation was not too complicated. :-)