Для обложки я купил плотную бумагу.

I

Breakdown of Для обложки я купил плотную бумагу.

• купить: to buy
• для: for
• бумага: the paper

Questions & Answers about Для обложки я купил плотную бумагу.

Why is обложки written with -и instead of обложка?

Because для requires the genitive case.

• Dictionary form: обложка = cover
• After для (for), it changes to genitive singular:
  • для обложки = for the cover / for a cover

So this is a very common pattern:

• для книги = for the book
• для мамы = for mom
• для обложки = for the cover

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What does для обложки mean exactly here?

It means for the cover or for a cover.

In this sentence, для expresses purpose: the paper was bought to be used for making a cover.

So the phrase answers the question:

• For what purpose did I buy the paper?
• For the cover.

Because Russian has no articles, для обложки could mean:

• for the cover
• for a cover

The exact English version depends on context.

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Why is it плотную бумагу, not плотная бумага?