Если аванс придёт вовремя, я наконец куплю новые кроссовки.

Because Russian and English handle future conditions differently.

In Russian, if the condition is about the future, you normally use a future form in the если clause:

• Если аванс придёт вовремя, я наконец куплю новые кроссовки.

This is completely standard. English uses the present after if, but Russian does not follow that pattern.

So a good rule is:

• English: If X happens, I will...
• Russian: Если X случится, я...

Придёт is the 3rd person singular future form of прийти (to come / to arrive).

This verb is irregular, so its forms are not built in a very transparent way from the infinitive. The future forms are:

• я приду
• ты придёшь
• он/она придёт
• мы придём
• вы придёте
• они придут

A useful thing to remember is that ё in придёт is important: it marks both the sound and the stress.

Because the sentence is talking about two single, completed events:

1. the advance payment arrives
2. I buy the sneakers

Russian uses perfective verbs for actions seen as complete, one-time results.

• прийти → придёт = to arrive, to come successfully
• купить → куплю = to buy, to complete the purchase

If you used imperfective verbs, the meaning would change. It could sound repeated, ongoing, or process-focused rather than a single finished event.

Because this is a real, possible condition, not a hypothetical or contrary-to-fact one.

The sentence means something like:

• If the advance arrives on time, I’ll finally buy new sneakers.

That is a normal future possibility, so Russian uses:

• если + future, then future

You use бы for unreal or hypothetical situations, for example:

• Если бы аванс пришёл вовремя, я бы купил новые кроссовки.

That means more like If the advance had come on time, I would have bought new sneakers or If it came on time, I would buy..., depending on context.

• аванс is nominative singular because it is the subject of придёт
• новые кроссовки is accusative plural because it is the direct object of куплю

So the structure is:

• Если аванс придёт вовремя = if the advance arrives on time
• я куплю новые кроссовки = I will buy new sneakers

A detail that often confuses learners: the accusative plural of inanimate nouns is the same as the nominative plural. That is why новые кроссовки looks the same here.

Compare:

• nominative: новые кроссовки
• accusative: новые кроссовки

Same form, different function.

Because вовремя is an adverb meaning on time or in time.

• вовремя = on time

This is different from во время, which is two words and usually means during:

• вовремя пришёл = arrived on time
• во время урока = during the lesson

This is a very common distinction in Russian, so it is worth memorizing.

Наконец means finally or at last.

It tells you that the speaker has wanted this for some time, or that this purchase has been delayed and is now expected to happen.

So:

• я куплю новые кроссовки = I’ll buy new sneakers
• я наконец куплю новые кроссовки = I’ll finally buy new sneakers

It adds an emotional nuance: relief, impatience, or long-awaited completion.

Yes, куплю already tells you the subject is I.

Russian often omits subject pronouns when they are obvious from the verb form. So this is also possible:

• Если аванс придёт вовремя, наконец куплю новые кроссовки.

That said, including я is very natural too. It can:

• make the sentence a little clearer
• add slight emphasis
• sound more balanced after a longer если clause

So я is not required, but it is perfectly normal here.

Because если аванс придёт вовремя is a subordinate clause introduced by если.

Russian normally separates subordinate clauses from the main clause with a comma:

• Если аванс придёт вовремя, я наконец куплю новые кроссовки.

If you reverse the order, the comma stays:

• Я наконец куплю новые кроссовки, если аванс придёт вовремя.

So the comma is required because of the clause structure.

Yes. Russian word order is flexible.

You can also say:

• Я наконец куплю новые кроссовки, если аванс придёт вовремя.

Both versions are correct. The difference is mainly one of emphasis:

• Если аванс придёт вовремя, ... puts the condition first
• Я наконец куплю..., если аванс придёт вовремя puts the main result first

The original version sounds very natural because it sets up the condition before giving the result.

Because кроссовки is plural, so the adjective must agree with it.

• новые = plural form of new
• кроссовки = sneakers

Russian adjectives agree with the nouns they describe in:

• gender
• number
• case

Here the noun is plural, so the adjective is plural too:

• новые кроссовки = new sneakers

That agreement is automatic in Russian grammar, so whenever you change the noun’s form, the adjective usually changes with it.