Questions & Answers about Эта ткань сильно мнётся, если положить её в сумку.
Because эта has to agree with ткань in gender, number, and case.
- ткань is feminine
- singular
- nominative here, because it is the subject of the sentence
So this fabric = эта ткань.
Compare:
- этот стол = this table
- эта ткань = this fabric
- это платье = this dress
- эти вещи = these things
Because ткань is the thing being described — the subject of the sentence.
The sentence is about what the fabric does:
- Эта ткань сильно мнётся = This fabric wrinkles a lot / gets very creased
So ткань stays in the nominative, which is the normal case for the subject.