Если спорить о пустяках, вечер пройдёт впустую.

This is a very common Russian pattern for making a general condition.

• Если спорить о пустяках... literally looks like If to argue about trifles...
• In natural English, that becomes something like:
  • If you argue about little things...
  • If people argue about trivial things...
  • When you spend time arguing about nonsense...

Using the infinitive here makes the statement feel general, impersonal, and a bit like an observation or warning, rather than talking about one specific person.

Compare:

• Если спорить о пустяках, вечер пройдёт впустую.
  = a general truth / observation

• Если вы будете спорить о пустяках, вечер пройдёт впустую.
  = directed at you specifically

So the infinitive is not weird here; it is a standard way to express “if one does X” in Russian.

Пустяках is the prepositional plural of пустяк.

The preposition о meaning about normally requires the prepositional case:

• о чём? = about what?
• о пустяке = about a trifle / a small thing
• о пустяках = about trifles / small matters

So:

• спорить о пустяках = to argue about trivial things

This is a useful pattern to remember:

• говорить о чём-то = to talk about something
• думать о чём-то = to think about something
• спорить о чём-то = to argue about something

Yes, it could be singular, but the plural sounds more natural here.

• о пустяках = about trivial things, little matters, small nonsense
• о пустяке = about one trivial thing

In this sentence, the idea is broad and general: people waste an evening by arguing over petty matters in general, not just one specific minor issue. That is why the plural fits better.

So:

• спорить о пустяках = a natural expression for bickering over little things
• спорить о пустяке = arguing over one minor thing

Here пройдёт is the future tense of the perfective verb пройти.

Russian often uses пройти for time passing:

• день прошёл = the day passed
• вечер прошёл = the evening passed
• время пройдёт = time will pass

In this sentence, the evening is viewed as one complete event, so the perfective verb is natural:

• вечер пройдёт впустую = the evening will pass / be spent in vain

Why not проходит?

• проходит is present tense and would mean something like is passing
• that does not fit this conditional result as well

Why not пройдётся?

• пройдётся is a different verb form and usually means something like will take a walk, will go over, or in some contexts will make cutting remarks
• it is not the right form for an evening passing

Впустую is an adverb meaning:

• in vain
• to no purpose
• wastefully
• for nothing useful

So вечер пройдёт впустую means the evening will be wasted.

A very literal sense is something like into emptiness, but in normal usage it simply means without a useful result.

Similar words:

• зря = for nothing, needlessly
• напрасно = in vain
• впустую = wastefully / to no good purpose

In this sentence, впустую works especially well because it fits with spending time badly:

• потратить время впустую = to waste time
• день прошёл впустую = the day was wasted

All three can relate to wasted effort or lack of result, but they are not always identical.

1. впустую

Focuses on something being wasted or spent without useful outcome.

• вечер пройдёт впустую = the evening will be wasted

This is the best fit here because the sentence is about wasting the evening.

2. зря

Very common, conversational. Means for nothing, needlessly.

• Мы зря спорили. = We argued for nothing.

3. напрасно

Often means in vain, unsuccessfully, sometimes a bit more formal.

• Он напрасно ждал. = He waited in vain.

In this sentence, впустую is especially natural because the subject is вечер, and the idea is that the time itself will be used up wastefully.

Russian often leaves the subject unstated when the meaning is general or obvious from context.

In Если спорить о пустяках..., there is no named subject because the sentence means something like:

• If you argue about trivial things...
• If people argue about trivial things...
• If one spends time arguing about little things...

This kind of subjectless, general construction is very normal in Russian.

The second clause does have a grammatical subject:

• вечер = the evening

So the structure is:

• Если спорить о пустяках = if people/you argue about trivial things
• вечер пройдёт впустую = the evening will be wasted

Because Если спорить о пустяках is a subordinate clause introduced by если (if), and it is followed by the main clause:

• Если спорить о пустяках, вечер пройдёт впустую.

This is the same basic idea as in English:

• If you argue about trivial things, the evening will be wasted.

Russian normally separates such conditional clauses with a comma.

You can also reverse the order:

• Вечер пройдёт впустую, если спорить о пустяках.

The comma is still needed.

Yes. Russian word order is flexible, though the original order is very natural.

Original:

• Если спорить о пустяках, вечер пройдёт впустую.

Possible alternative:

• Вечер пройдёт впустую, если спорить о пустяках.

Both mean the same thing.

The difference is mainly in focus:

• Если спорить о пустяках... puts the condition first
• Вечер пройдёт впустую... puts the result first

The original sounds like a warning that begins with the cause.

Спорить is imperfective because the sentence refers to the process/activity of arguing, not to a single completed argument.

• спорить = to argue, to be arguing
• поспорить = to argue for a while / to have an argument (perfective)

Here the meaning is general:

• If you spend time arguing about trivial things...

That naturally calls for the imperfective, because it describes an ongoing activity that fills the evening.

If you used a perfective form, the nuance would change and sound less natural here.

It is not a rigid idiom, but it is a very natural collocation.

• спорить commonly goes with о + prepositional
• пустяки commonly refers to trifles, petty matters, small unimportant things

So спорить о пустяках is a normal way to say:

• to argue about trifles
• to bicker over little things
• to argue about nothing important

It sounds natural and idiomatic, even if it is not a fixed phrase you must memorize as one chunk.

Yes.

• пройдёт is pronounced roughly pray-DYOT
• the ё always represents a yo sound

In writing, Russian sometimes omits the dots over ё, especially in casual or standard printed text, so you may also see:

• пройдет

But it is still pronounced пройдёт, not пройдет with plain e.

For learners, it is helpful to remember the dictionary form:

• пройти
• пройдёт

Sometimes, yes, depending on style and context.

Если literally means if, but in general statements like this, English may naturally use:

• If you argue about trivial things, the evening will be wasted.
• When you argue about trivial things, the evening ends up being wasted.

The Russian sentence presents a general cause-and-result relationship. English often uses if for that, but when can also work if you want to stress that the result typically happens whenever the condition occurs.

Still, if is the most direct match for если.