Иногда ответ приходит мгновенно, а порой приходится ждать до вечера.

Yes, both can mean sometimes, but they are not completely identical in feel.

• иногда = the most neutral, common word for sometimes
• порой = at times / sometimes, often a bit more literary or expressive

Using both in one sentence avoids repetition and gives the sentence a more natural style. So:

• Иногда ответ приходит мгновенно = Sometimes the answer comes instantly
• а порой приходится ждать до вечера = and at other times you have to wait until evening

Here порой helps create a contrast: sometimes this happens, but at other times that happens.

In this sentence, а marks a contrast between two situations.

• и = and, simple addition
• но = but, stronger contradiction
• а = often and/but/whereas, showing comparison or contrast

So here:

• Иногда ответ приходит мгновенно, а порой приходится ждать до вечера.

means something like:

• Sometimes the answer comes instantly, whereas at other times you have to wait until evening.

It is not a harsh contradiction, so а sounds more natural than но.

Russian often uses the present tense to describe things that happen generally, habitually, or as a regular possibility.

So ответ приходит мгновенно does not mean only the answer is coming right now. It means:

• the answer comes instantly
• the answer arrives instantly
• the answer sometimes comes instantly

This is the same kind of present tense as in English sentences like:

• Water boils at 100°C.
• Sometimes inspiration comes quickly.

It describes a general pattern.

Because ответ is the grammatical subject of приходит.

In ответ приходит:

• ответ = the answer
• приходит = comes / arrives

So literally, the answer comes. Since it is the subject, it stays in the nominative case.

Yes — this is a normal metaphor in Russian, just as English can say:

• the answer came to me
• the idea came suddenly

Russian often uses verbs like приходить (to come / arrive) for thoughts, answers, ideas, inspiration, and realizations.

So ответ приходит мгновенно is very natural and idiomatic.

Мгновенно is an adverb meaning:

• instantly
• in an instant
• immediately

It describes how the answer comes.

Related forms:

• мгновение = moment
• мгновенный = instantaneous
• мгновенно = instantly

So:

• ответ приходит мгновенно = the answer comes instantly

Приходится is a very common Russian impersonal verb meaning:

• one has to
• it becomes necessary to
• someone ends up having to

In this sentence:

• приходится ждать = (one/you/we) have to wait

This verb is often used when the situation forces something, rather than when a person chooses it freely.

Compare:

• Я должен ждать = I must wait / I am obliged to wait
• Мне приходится ждать = I have to wait / I end up having to wait

In your sentence, the subject is not stated explicitly, so it feels general: sometimes you just have to wait.

Because this is an impersonal construction.

Russian often leaves the experiencer unstated when the meaning is general. In English, we might translate it with:

• you have to wait
• one has to wait
• people have to wait

So:

• порой приходится ждать до вечера

means roughly:

• sometimes you have to wait until evening
• sometimes one has to wait until evening

The sentence is about a general experience, not one specific person.

Because приходится is followed by an infinitive that tells you what has to be done.

Pattern:

• приходится + infinitive

Examples:

• приходится работать = have to work
• приходится искать = have to look for
• приходится ждать = have to wait

So in this sentence:

• приходится ждать = have to wait

Because the preposition до requires the genitive case.

• nominative: вечер
• genitive: вечера

So:

• до вечера = until evening / until the evening

This is a fixed and very common pattern:

• до утра = until morning
• до ночи = until night
• до понедельника = until Monday

Usually until evening is the most natural translation.

Russian often uses expressions like до вечера, до утра, до ночи without an article, and English usually translates them without one too.

So:

• ждать до вечера = wait until evening

Depending on context, English might also say wait until the evening, but until evening is the most direct and natural choice.

The comma separates two clauses joined by а.

The sentence has two parts:

1. Иногда ответ приходит мгновенно
2. а порой приходится ждать до вечера

Russian normally uses a comma before а when it connects two clauses like this.

So the comma is required here.

Yes. Russian word order is flexible, though the original order sounds natural and balanced.

Original:

• Иногда ответ приходит мгновенно, а порой приходится ждать до вечера.

Possible variations:

• Ответ иногда приходит мгновенно, а порой приходится ждать до вечера.
• Порой приходится ждать до вечера, а иногда ответ приходит мгновенно.

These are grammatical, but they shift emphasis slightly.

The original version is elegant because it contrasts two adverbs right away:

• Иногда ...
• порой ...

That makes the sentence flow well.

Not exactly a fixed idiom, but it is a very natural combination.

• порой = sometimes / at times
• приходится = have to
• ждать = wait

Together they form a very common type of phrase meaning:

• at times you have to wait

So even if it is not a special idiom, it is absolutely standard Russian.

It sounds neutral, natural, and slightly polished.

A few things create that effect:

• порой is a bit more literary than plain иногда
• the contrast with а makes the sentence flow smoothly
• ответ приходит sounds slightly figurative and elegant

So the sentence feels like good standard Russian, not slang and not overly formal.