Questions & Answers about Это красивая безделушка.
Because это here does not directly modify безделушка.
In Это красивая безделушка, это means this is... and stays in the fixed form это.
Compare:
- Это красивая безделушка. = This is a pretty trinket.
- Эта красивая безделушка... = This pretty trinket...
In the second example, эта directly goes with the feminine noun безделушка, so it has to agree with it.
Because it agrees with безделушка, which is:
- feminine
- singular
- nominative
Russian adjectives change their endings to match the noun they describe.
So you get:
- masculine: красивый
- feminine: красивая
- neuter: красивое
- plural: красивые
Since безделушка is feminine singular, красивая is the correct form.