Basic Russian spelling rules

Abt Russian spelling rules I see in Aphadictionary what follows:

"Table I: Russian Vowels

Hard
Soft Sound
(roughly)
а я 'Ah!'
э е 'Eh?'
ы и 'Eee!'
о ё 'Oh!'
у ю 'Ooo!'


The vowel letters in the left column occur only after 'hard' consonants; those in the right column appear only after 'soft' consonants.

You must remember that, after applying all the other rules of Russian, especially when adding noun, verb, and adjectives endings, you must be sure to convert any 'hard consonant' vowel to its 'soft' alternate, if the suffix begins with a vowel and is added to a stem ending on a consonant.

Also, if any word ends on a vowel from the right-hand column, then its stem ends on a soft consonant, so when changing the endings on such words, you must consistently use the vowels from the 'soft' column. The following table shows what I mean. [For best results in studying this table, focus on one word at a time and follow the change in each word. Begin with the forms in the left columns; they provide you with the basic vowel used after hard consonants. The right-hand column shows you the rule for converting the basic vowel to the vowel used after a soft consonant.]
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Table II: Vowel Alternations after Soft Consonants
after hard
consonants Examples after soft
consonants Examples
C + а стол-а Cь + а > я читатель-а
C + e стол-е Cь + е > е
C + ы стол-ы Cь + ы > и
C + о окн-о Cь + о > ё
C + у стол-у Cь + у > ю читатель-у"

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Everything is clear save what I have quoted in bold:You must remember that, after applying all the other rules of Russian, especially when adding noun, verb, and adjectives endings, you must be sure to convert any 'hard consonant' vowel to its 'soft' alternate, if the suffix begins with a vowel and is added to a stem ending on a consonant. For example, if I add an 'a' to окн-о I will write окн-а and not окн-я. Perhaps it should be said : if the suffix begins with a vowel and is added to a stem ending on a soft consonant. It is true that in locative окн-e, стол-е a softening of the previous hard consonant is made. I don't understand. Can you make it clear for me? Thanks.

Quote:

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if the suffix begins with a vowel and is added to a stem ending on a consonant.

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I don't understand this phrase too. And looks like they confused suffix and ending.

The rule is: consonants before vowels from the right column are softened. That is all.

Thank you. Nor I couldn't get anyrhing of it. Surely the author knew what was in his mind but expressed it rather confusely.