Hardness and softness
We can pronounce them as soft or hard sounds. These sounds form pairs of soft(') and hard sound. The same letter can be as soft as hard. The softness - hardness of paired sounds denote the letter which is following the particular consonant. А, У, О, Э, Ы - make the consonant before them hard; Я, Ю, Ё, Е, И - soft. For example: рад - ряд, пыл - пил, лук - люк. At the end of the letter and between consonants the softness denote letter Ь (soft sign). For example: кон - конь; кров - кровь; клад - кладь; банка - банька; толк - только.
Hard and soft consonants
Paired and unpaired
You can split consonants into two groups - paired and unpaired sounds. The first group (paired sounds) contains of two categories: voiced - voiceless (unvoiced) sounds pairs and soft - hard sounds pairs. The second group contains unpaired sounds: soft, hard, voiced and voiceless.
Soft-hard unpaired sounds
Soft - hard unpaired sounds in Russian language don't require any letter after to denote their softness or hardness. So, it's not necessarily to write Ы after Ж, Ш to show their hardness and also it's not necessarily to write Я, Ю after Ч, Щ to show their softness. That's why we write А, У, И after these letters. For example: жар, шар, час, пощада; жук, шум, чудо, щука; жир, камыши, чин, щит.
Voiced and voiceless sounds in Russian language
How we pronounce sounds partly depends on their place in the word. For example - voiced sounds at the end of the word becomes voiceless (дуб). Also it happens if the following consonant is voiceless (пруд, будка). The same way voiceless sounds could become by voiced if the next sound is voiced (молотьба).
Voiced and unvoiced consonants
Consonant sound J
There are few ways to convey consonant sound 'J' in Russian language. Тhe first one when we use letter Й. Letter Й comes after vowels (at the end of the word or before consonant). For example: рай, рой, майка, мойка. There is one more way to convey sound J, pronouncing letters Ю, Я, Ё, Е we produce this sound 'J'. In these cases sound 'J' comes before vowel - at the beginning of the word and after vowels. For example: юг, яблоко, боязнь, ёлка. In such cases after consonant and before vowels Ю, Я, Ё, Е we write letters Ъ or Ь, which helps to break down these sounds. For example: предъюбилейный, объятие, подъехал, подъём; друзья, вьюга, ружьё. After Ь with the same function (to convey sound J) can be also endowed letter И (соловьи).