About "ь" and "ъ"

These comments were originally written by Shady_arc on Duolingo Russian forum.

The alphabet has two special characters (which ~1000 years ago used to be short vowels).

• the soft sign ь marks the palatalisation of the consonant, i.e. the middle of your tongue is raised during the pronunciation (close to how you pronounce "ee" as in "meet"). Watch THIS if you don't get it. A few examples include тень, лошадь, мальчик, альбом, только, здесь
• we also use ь to signal that е, я, ё, ю retain their [j], e.g., in семья, льёт, пью. This is also how ьо is pronounced in loanwords like бульон and медальон.
• we also use ь after some perma-soft of perma-hard consonants as a spelling convention. For example, мышь "mouse" or ты сидишь "you are sitting" have it; Ш is non-palatalised in modern Russian.
• the hard sign ъ is used between a prefix and е / я / ё / ю of the root: these letters will be pronounced with an initial [j] and will not generally palatalise the preceding consonant. You can see it in съесть, въезд, объём, съёмка (and the same in compounds like трёхъярусный) as well as in a small number of loanwords with recognised prefixes (e.g., объект). This is a rare letter.

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Ъ is used to separate a prefix (or root) ending in a consonant from a root that starts with е, ё, ю or я, e.g. in объём "volume" or четырёхъядерный "quad-core"

You pronounce the vowel with a Й. For example, объём does not use the typical pronunciation of ё after a consonant—instead you pronounce "yo" like at the beginning of a word (i.e. /ɐbjɵm/).

Perfective verbs сесть "sit" and съесть "eat" provide a good demonstration of the difference: one has the "y" sound after "s", the other does not. For speakers who palatalise the first "s", the rest of the sounds are identical.

So, the hard sign is ONLY used at the morpheme boundary (in modern orthography) and has no other purpose. Most commonly, you see it after a prefix (e.g., съесть, съем, отъехать, подъезд, въехать, взъерошить etc.), including some words of foreign origin (e.g., объект "object").