Мяч с нижним вращением трудно вернуть.

Not really. In this sentence, мяч is the thing being returned, so logically it is the object of вернуть.

What confuses learners is that мяч looks like a nominative form. But for a masculine inanimate noun, the accusative singular is the same as the nominative singular:

• nominative: мяч
• accusative: мяч

So in Мяч с нижним вращением трудно вернуть, мяч is still the object of вернуть, just moved to the front for emphasis.

A more neutral word order would be:

• Трудно вернуть мяч с нижним вращением.

Both mean the same thing.

Because they come after the preposition с meaning with, and in this meaning с normally takes the instrumental case.

Base form:

• нижнее вращение = lower spin / underspin / backspin

After с:

• с нижним вращением = with lower spin

So:

• вращение → вращением
• нижнее → нижним

The adjective changes to match the noun in case, number, and gender.

Literally, it is with lower spin, but in natural sports English it usually means:

• with backspin
• with underspin

So Мяч с нижним вращением трудно вернуть is naturally understood as something like:

• A backspin ball is hard to return.
• A ball with underspin is hard to return.

Трудно here is an adverb/predicative word meaning it is difficult.

Russian often uses this pattern:

• трудно + infinitive
• легко + infinitive
• интересно + infinitive
• полезно + infinitive

So:

• трудно вернуть = it is difficult to return

This is very natural Russian. English uses a dummy subject it; Russian usually does not need one.

Вернуть is the perfective verb, and here it means to return successfully as a single completed action.

In sports, that makes sense: you are talking about managing to return the ball.

• вернуть = to return it successfully / get it back
• возвращать = to be returning / to return repeatedly or in a more general process sense

So трудно вернуть means hard to return in the sense of hard to pull off successfully.

Also, вернуть обратно would usually be redundant, because вернуть already contains the idea of returning something back.

Yes. Russian word order is flexible.

A neutral version is:

• Трудно вернуть мяч с нижним вращением.

The given version:

• Мяч с нижним вращением трудно вернуть.

puts мяч с нижним вращением first as the topic, as if saying:

• As for a ball with backspin, it’s hard to return.

So the change is mostly about emphasis and information structure, not basic meaning.

Because Russian has no articles.

So мяч can mean:

• a ball
• the ball

Which one is meant depends on context.

In a general statement like this, English often uses:

• A ball with backspin is hard to return or
• The ball with backspin is hard to return

Russian leaves that choice to context.

Russian does not need a dummy subject like English it in sentences of this type.

English:

• It is hard to return a ball with backspin.

Russian:

• Трудно вернуть мяч с нижним вращением.

So трудно can stand on its own with an infinitive, without any separate subject.

Yes, especially in sports contexts such as table tennis or tennis. It refers to underspin/backspin.

Depending on the sport or style, you may also hear other expressions, but нижнее вращение is a normal and understandable term.

So:

• мяч с нижним вращением = a ball with backspin / underspin

A good way to break it down is:

• Мяч с нижним вращением = a ball with backspin
• трудно вернуть = is hard to return

So the whole sentence is literally something like:

• A ball with backspin [is] hard to return.

Or in more natural English:

• A ball with backspin is hard to return.
• It is hard to return a ball with backspin.

Both are good translations of the Russian structure.