Siła wiatru jest duża.

Because wiatru is the genitive singular form of wiatr.

In siła wiatru, Polish is using a structure that literally means the strength of the wind. After a noun like siła (strength), the thing being measured or described often goes into the genitive case.

So:

• wiatr = wind
• wiatru = of the wind

This is very common in Polish:

• kolor domu = the color of the house
• koniec dnia = the end of the day
• siła głosu = the strength of the voice

Because duża has to agree with siła, and siła is a feminine singular noun.

In Polish, adjectives change their form to match the noun in:

• gender
• number
• case

Here:

• siła is feminine
• the sentence describes siła
• so the adjective must be feminine too: duża

Compare:

• duży wiatr = strong/big wind? (grammatically masculine, though not the most natural phrase)
• duża siła = great strength

Jest means is. It is the 3rd person singular form of the verb być (to be).

So the structure is:

• Siła wiatru = the strength of the wind
• jest = is
• duża = large/great

Together: The strength of the wind is great/strong/high.

Sometimes, yes, but not in the most neutral full sentence.

In everyday Polish, especially in informal speech, headlines, notes, or poetic style, people sometimes omit jest:

• Siła wiatru duża.

But the standard, complete sentence is:

• Siła wiatru jest duża.

So as a learner, it is safest to use jest here.

It can do both.

Polish often uses noun-based expressions where English might prefer an adjective with a subject. So both of these are natural, depending on context:

• Siła wiatru jest duża. = The strength of the wind is great/high.
• Wiatr jest silny. = The wind is strong.

The first sounds a bit more formal, technical, or descriptive, and you might see it in weather reports or measurements. The second is simpler and more conversational.

Yes, duży/duża/duże usually means big or large, but with abstract nouns like siła it often means great, high, or strong in effect.

So:

• duża siła = great strength / high force

That is why Siła wiatru jest duża sounds natural even though English would often say The wind is strong rather than The strength of the wind is big.

Siła is in the nominative singular.

It is the subject of the sentence, so nominative is expected. The sentence is built like this:

• Siła — subject, nominative
• wiatru — dependent noun, genitive
• jest duża — predicate

So the main noun is siła, and the adjective duża agrees with siła, not with wiatru.

Yes. Polish word order is more flexible than English word order, though not completely free.

The neutral order here is:

• Siła wiatru jest duża.

But you might also hear:

• Duża jest siła wiatru.
• Jest duża siła wiatru.

These alternatives change the emphasis a little. As a learner, the original order is the safest and most neutral.

A rough pronunciation guide is:

• Siła ≈ SHEE-wah
• wiatru ≈ VYAH-troo
• jest ≈ yest
• duża ≈ DOO-zhah

A more careful approximation: SHEE-wa VYAH-troo yest DOO-zha

A few useful notes:

• si before a vowel sounds like a soft sh-like sound
• w in Polish sounds like English v
• j sounds like English y
• ż sounds like the s in measure

In Polish, stress usually falls on the second-to-last syllable of each word.

So:

• SI-ła → stress on Si
• wia-tru → stress on wia
• jest → one syllable, so naturally stressed when needed
• DU-ża → stress on du

So the sentence rhythm is roughly: SIła WIAtru jest DUża

You usually would not say that.

Although silny/silna/silne means strong, it normally describes something that itself has strength, rather than describing the amount of strength. In Polish, siła is more naturally described as:

• duża = great/high
• mała = small/low

So:

• Siła wiatru jest duża. = natural
• Siła wiatru jest silna. = much less natural

A better alternative with silny is:

• Wiatr jest silny. = The wind is strong.

Yes, especially in more formal or technical language.

In weather reports, Polish often uses nouns like:

• siła wiatru = wind strength
• prędkość wiatru = wind speed
• temperatura = temperature

So Siła wiatru jest duża sounds more like measured or report-style language, while Wiatr jest silny sounds more everyday and conversational.