Questions & Answers about Эта диета кажется сложной, но полезной.
Why is it эта диета and not эту диету at the beginning?
Because эта диета is the subject of the sentence, so it must be in the nominative case.
- эта – nominative feminine singular demonstrative pronoun (this)
- диета – nominative feminine singular noun (diet)
The verb кажется here behaves like “to be / to seem” in English. The thing that seems is the subject:
- Эта диета кажется... – This diet seems...
If you used эту диету (accusative), it would normally be the object of some action, for example:
- Я начинаю эту диету. – I’m starting this diet.
But in our sentence, nothing is being done to the diet; the diet itself seems a certain way, so nominative is correct: эта диета.
Why are сложной and полезной in that form? Why not just сложная, но полезная?