Kość jest twarda.

Because kość is a feminine singular noun, and Polish adjectives must agree with the noun in gender, number, and case.

So:

• twardy = masculine singular
• twarda = feminine singular
• twarde = neuter singular or some plural uses

Since kość is feminine, the correct form is twarda.

Yes. In Polish, many feminine nouns end in -a, but not all of them do.
A lot of feminine nouns end in a soft consonant or have the -ść ending, and kość is one of them.

So although it may look unusual at first, kość is grammatically feminine.

It is in the nominative singular.

In Kość jest twarda, the noun kość is the subject of the sentence, so nominative is exactly what you would expect.

The adjective twarda is also in the nominative singular because it is describing the subject through jest.

With być (to be) plus an adjective, Polish normally uses the nominative form of the adjective.

So:

• Kość jest twarda = correct
• Kość jest twardą = incorrect in this meaning

The form twardą is accusative/instrumental feminine singular, and it does not belong here.

Jest is the 3rd person singular of być (to be), so it means is.

In normal standard Polish, you usually keep it:

• Kość jest twarda. = The bone is hard.

Omitting jest would sound incomplete in an ordinary sentence.
In some special styles, like notes, headlines, or very poetic language, words may be dropped, but for a learner, it is best to use jest here.

Polish does not have articles like English a/an and the.

So Kość jest twarda could mean:

• The bone is hard
• A bone is hard
• Bone is hard / Bones are hard in a general sense, depending on context

Context tells you which meaning is intended.

A rough English-friendly guide is:

kość jest twarda ≈ koshch yest TVAR-da

A few notes:

• kość has a final sound cluster that does not exist naturally in English
• jest sounds like yest
• twarda starts with a consonant cluster, roughly tvar-da
• Polish stress is usually on the next-to-last syllable
  • KOŚĆ is one syllable
  • jest is one syllable
  • twar-da → stress on twar

If you want a more technical pronunciation:

• kość = /kɔɕt͡ɕ/
• jest = /jɛst/
• twarda = /ˈtfarda/

Yes. Polish word order is more flexible than English because endings carry a lot of grammatical information.

The neutral order is:

• Kość jest twarda.

But other orders are possible for emphasis, for example:

• Twarda jest kość. = more literary or emphatic
• Jest twarda kość. = unusual in isolation

For learners, Kość jest twarda is the safest and most natural version.

You add nie before jest:

• Kość nie jest twarda. = The bone is not hard.

This is the normal way to negate a sentence with być.

The plural would be:

• Kości są twarde. = Bones are hard.

Changes:

• kość → kości = plural
• jest → są = are
• twarda → twarde = plural adjective form

This again shows agreement: the adjective changes to match the noun.

Not always. Kość can also mean die (the singular of dice) in some contexts.

So depending on context:

• kość = bone
• kość do gry = die / gaming die

In Kość jest twarda, the most natural meaning is bone, especially with twarda meaning physically hard.