С ним бесполезно спорить.

Because the preposition с requires the instrumental case here.

• он = he (nominative)
• его = him / his (accusative or genitive)
• ним = him (instrumental)

So:

• с ним = with him

In Russian, many prepositions force a certain case, and с often takes the instrumental when it means with someone.

Literally, yes: with him.

But in English, with the verb argue, we usually say argue with him, so the phrase translates naturally as:

• It’s useless to argue with him.

Russian uses спорить с кем-то for to argue with someone / to dispute with someone.

So с ним is the normal way to say with him after спорить.

Here бесполезно means it is useless.

It comes from бесполезный (useless), but in this sentence it functions as a predicative word in an impersonal sentence. In practice, you can think of it as meaning:

• it is useless
• there’s no point

So:

• С ним бесполезно спорить. = It is useless to argue with him.

This is a very common Russian pattern:

• Трудно понять. = It is hard to understand.
• Важно помнить. = It is important to remember.
• Невозможно сделать. = It is impossible to do.

Because Russian often uses this structure:

• [predicative word] + infinitive

So here:

• бесполезно спорить = it is useless to argue

The infinitive expresses the action in a general way, without naming a subject like I, you, or they.

This makes the whole sentence feel general:

• Arguing with him is pointless
• It’s useless to argue with him

Not in Russian.

English usually needs a subject, so we say:

• It is useless to argue with him.

But Russian can make this kind of statement without any subject at all. That is why this is called an impersonal sentence.

So Russian simply says:

• С ним бесполезно спорить.

There is no word corresponding to English it.

Because спорить is the imperfective verb, and it fits best when talking about an action in general.

• спорить = to argue, to be arguing, to argue in general
• поспорить = to argue for a while / to have an argument once

In this sentence, the speaker means something general:

• Arguing with him is pointless
• There is no point in arguing with him

That is why спорить sounds natural.

If you said С ним бесполезно поспорить, it would sound odd in most contexts, because поспорить suggests a more specific, bounded event.

Yes. Russian word order is flexible, and several versions are possible:

• С ним бесполезно спорить.
• Бесполезно спорить с ним.
• Спорить с ним бесполезно.

All mean roughly the same thing.

The differences are mostly about focus and emphasis:

• С ним бесполезно спорить.
  Focuses early on with him.
• Бесполезно спорить с ним.
  Starts with it’s useless, which sounds very natural.
• Спорить с ним бесполезно.
  Highlights the whole action arguing with him.

Your original sentence is completely natural.

Usually not in the same way.

Russian normally says:

• С ним бесполезно спорить.
• Бесполезно спорить с ним.

Adding это often sounds unnecessary or changes the structure. Russian does not need a dummy subject like English it.

You might hear это бесполезно in a different context, where это really refers to something specific:

• Пытаться его переубедить — это бесполезно.
  Trying to change his mind is useless.

But for your sentence, the version without это is the standard one.

Yes, depending on context.

The verb спорить can mean:

• to argue
• to dispute
• to disagree

So the sentence can carry slightly different English translations:

• It’s useless to argue with him.
• There’s no point in disputing with him.
• It’s no use disagreeing with him.

The exact nuance depends on the situation, but argue with him is the most straightforward translation for learners.

Only the pronoun changes, and it stays in the instrumental case after с:

• С ней бесполезно спорить. = It’s useless to argue with her.
• С ними бесполезно спорить. = It’s useless to argue with them.
• С тобой бесполезно спорить. = It’s useless to argue with you.
• С вами бесполезно спорить. = It’s useless to argue with you (formal/plural).
• Со мной бесполезно спорить. = It’s useless to argue with me.

Notice that sometimes с becomes со, as in со мной. That is just a pronunciation-friendly variant.

The stress is:

• С ним бесполе́зно спори́ть.

A rough pronunciation guide:

• с ним = s neem
• бесполе́зно = byes-pa-LYEZ-na
• спори́ть = spa-REETʹ

Two important stress points:

• бесполе́зно → stress on ле
• спори́ть → stress on the last syllable

If you want to sound natural, keep the sentence flowing smoothly as one idea:

• С ним бесполе́зно спори́ть.