Если мы будем спорить дальше, мы только потратим время впустую.

That is a very common point of confusion for English speakers.

In Russian, если can be followed by a future verb when the situation really refers to the future:

Если мы будем спорить дальше, ...

This is completely normal Russian. English and Russian handle this differently:

• English: If we keep arguing, we'll only waste time.
• Russian: Если мы будем спорить дальше, мы только потратим время впустую.

So Russian does not follow the same rule as English here. You do not need to switch to a present tense after если just because English does.

Будем спорить is the future of the imperfective verb спорить. It emphasizes an ongoing or continuing action:

• будем спорить = will be arguing / will keep arguing

That fits the idea of continuing the argument.

A perfective form such as поспорим would sound more like a single bounded event, something like:

• we’ll have an argument
• we’ll argue for a bit

But here the speaker means if we go on arguing further, so the imperfective будем спорить is the natural choice.

Here дальше means further, any longer, or from this point on.

So:

• спорить дальше = to argue further
• если мы будем спорить дальше = if we keep arguing / if we continue arguing

It does not literally mean physical distance here. It is about continuing an action.

This is about verbal aspect.

• тратить = imperfective, focuses on the process
• потратить = perfective, focuses on the completed result

In this sentence, the speaker is talking about the outcome:

• we’ll end up wasting time
• the result will be wasted time

That is why потратим is used. It presents the wasting of time as a complete result of continuing the argument.

If you said будем тратить время, it would focus more on the ongoing process of wasting time, not so much on the final consequence.

Here только means only in the sense of all that will happen is...

So:

• мы только потратим время впустую = we’ll only waste time / all we’ll do is waste time

It limits the result. The idea is:

• nothing useful will come of it
• the only consequence will be wasted time

Время is the direct object of потратим, so it is in the accusative case.

However, время is a neuter noun, and for many neuter nouns the accusative singular looks exactly the same as the nominative singular:

• nominative: время
• accusative: время

So the case does change grammatically, but the form happens to stay the same.

Впустую is an adverb meaning:

• in vain
• for nothing
• to no purpose
• uselessly

So:

• потратить время впустую = to waste time
• more literally, to spend time for nothing

This is a very natural Russian expression. It reinforces the idea that the time will be spent without any useful result.

Yes, you often can, but the nuance is a little different.

• впустую stresses that something is spent or used with no useful result
• зря means for nothing, needlessly, in vain

So both are possible in many contexts, but впустую works especially well with things like time, effort, money:

• потратить время впустую
• потратить деньги впустую

It sounds very idiomatic here.

Yes, it could be omitted:

Если мы будем спорить дальше, только потратим время впустую.

That is possible and natural.

But repeating мы is also normal. It can make the sentence clearer, more balanced, or slightly more emphatic:

• Если мы будем спорить дальше, мы только потратим время впустую.

So the repeated мы is not required, but it does not sound wrong at all.

Because the sentence has two clauses:

1. Если мы будем спорить дальше
2. мы только потратим время впустую

In Russian, a subordinate clause introduced by если is separated from the main clause by a comma.

So the comma here is standard punctuation.

Yes. Russian word order is fairly flexible, and changing it can shift emphasis.

For example:

• Если мы будем спорить дальше, мы только потратим время впустую.
  Neutral, standard order.

• Если мы будем спорить дальше, только время потратим впустую.
  Slightly more emphasis on time as the thing being wasted.

• Мы только потратим время впустую, если будем спорить дальше.
  Same basic meaning, but the main result comes first.

So the original version is a very natural neutral way to say it, but other orders are possible depending on emphasis.