Taşı kaldırmak zor.

Because taşı has the accusative ending. In Turkish, a definite direct object often takes this ending.

• taş = stone / a stone
• taşı = the stone (as the object of the verb)

So:

• Taş kaldırmak zor. = Lifting stone(s) is hard / Stone-lifting is hard / It is hard to lift a stone
• Taşı kaldırmak zor. = Lifting the stone is hard / It is hard to lift the stone

Here, the sentence is talking about a specific stone, so taşı is used.

Kaldırmak is the infinitive form of the verb, meaning to lift, to raise, or sometimes to remove, depending on context.

In this sentence, it means to lift.

The ending -mak / -mek is the normal Turkish infinitive ending, similar to English to ...:

• gelmek = to come
• gitmek = to go
• kaldırmak = to lift

So taşı kaldırmak means to lift the stone.

In Turkish, the verb to be is often not expressed in simple present-tense sentences like this.

So:

• Taşı kaldırmak zor.

literally looks like:

• Lifting the stone difficult.

But it means:

• Lifting the stone is difficult.

This is very normal in Turkish. The idea of is is understood automatically.

Zor is an adjective meaning difficult, hard, or tough.

In this sentence, it works as the predicate:

• Taşı kaldırmak = lifting the stone
• zor = difficult

So the whole sentence means:

• Lifting the stone is difficult
• It is hard to lift the stone

Yes. The whole infinitive phrase taşı kaldırmak functions like a noun phrase and acts as the subject.

A useful literal way to think about it is:

• Taşı kaldırmak zor.
• The lifting of the stone is difficult.

Turkish often uses infinitives this way, just like English can use lifting the stone as the subject.

Because in Turkish, objects usually come before the verb.

Even though kaldırmak is an infinitive here, it still behaves verbally enough that its object comes before it:

• taşı kaldırmak = to lift the stone

This is the normal Turkish order.

So while English says:

• to lift the stone

Turkish says:

• the stone to lift

That is completely standard.

Yes. That is a very natural English translation.

Turkish:

• Taşı kaldırmak zor.

Possible English translations:

• Lifting the stone is difficult.
• It is difficult to lift the stone.
• It is hard to lift the stone.

These all express the same basic meaning.

The difference is mainly definiteness.

• Taş kaldırmak zor = Lifting stone(s) is hard / It is hard to lift a stone
• Taşı kaldırmak zor = Lifting the stone is hard

So:

• taş is more general or indefinite
• taşı refers to a specific stone

This is a very important difference in Turkish.

The given order is the most natural and neutral one:

• Taşı kaldırmak zor.

Turkish word order is somewhat flexible, but not all versions sound equally natural.

This sentence is built as:

• [object + infinitive] + predicate
• taşı kaldırmak
  • zor

That is the clearest and most standard way to say it.

A few pronunciation points may help:

• ş is pronounced like English sh
• ı is a Turkish vowel that has no exact English equivalent; it is a relaxed, unrounded vowel, somewhat like the vowel in sofa but farther back
• zor sounds roughly like zor with a clear r at the end

So:

• taşı ≈ tah-shuh (very approximate)
• zor ≈ zor

Also note that Turkish pronunciation is usually quite regular once you know the letters.