Questions & Answers about У мачехи есть близнецы.
Since this lesson is about мать, shouldn't мачеха (stepmother) also have an irregular ending like у мачери?
No, мачеха is a standard feminine noun. The special -ер- stem added to the oblique cases of мать (like the Genitive у матери) and дочь does not apply here. You simply decline мачеха like any regular noun ending in -а.
Why does the Genitive form of мачеха end in -и instead of -ы?
The preposition у triggers the Genitive case. Normally, feminine nouns ending in -а change to -ы in the Genitive. However, the 7-letter spelling rule states you can never write -ы after the letter х (as well as к, г, ж, ч, ш, щ). Therefore, the ending must be -и, giving us мачехи.
Why does близнецы end in -ы? Doesn't the letter ц have a spelling rule too?
It does, but it behaves differently! The letter ц is not part of the 7-letter spelling rule. For noun endings after ц, Russian requires -ы instead of -и. That's why the plural of близнец (twin) is близнецы, just like отец (father) becomes отцы.