Я куплю кроссовки получше, потому что в старых уже неудобно бегать.

Куплю is the perfective future of купить, so it presents the action as a single completed result: I’ll buy (a pair).

That fits the sentence well, because the speaker means a concrete future decision with an expected outcome.

• Я куплю кроссовки = I’ll buy sneakers / I’m going to buy sneakers
• Я буду покупать кроссовки would sound more like I’ll be buying sneakers, I’ll buy them over some period, or it may focus on the process rather than the finished purchase.

So куплю is the natural choice here.

In this sentence, кроссовки is the direct object of куплю, so it is in the accusative case.

But for inanimate plural nouns, the accusative looks the same as the nominative:

• nominative: кроссовки
• accusative: кроссовки

So the form does not visibly change.

This is very common in Russian:

• Я вижу столы = I see tables
• Я куплю кроссовки = I’ll buy sneakers

Получше is a colloquial comparative form meaning better, often with a softer nuance like a bit better, somewhat better, or better ones.

Compare:

• лучше = better
• получше = a little better / nicer / better ones

In real speech, получше often sounds more natural when someone means they want an improvement, not necessarily the absolute best possible option.

So here:

• кроссовки получше = better sneakers / a better pair of sneakers

It is less formal and often more conversational than plain лучше.

In Russian, comparatives like лучше, хуже, получше, поменьше, побольше often come after the noun they describe.

So:

• кроссовки получше = better sneakers
• вариант получше = a better option
• место получше = a better place

This word order is very natural in Russian.

You may also see other arrangements in Russian, because word order is flexible, but noun + comparative is especially common in this kind of everyday statement.

В старых literally means in the old ones.

The noun кроссовках is omitted because it is already obvious from context. The full version would be:

• в старых кроссовках

Russian often leaves out a noun when an adjective clearly points to it. This is similar to English the old ones.

So:

• в старых уже неудобно бегать means
• it’s already uncomfortable to run in the old ones

Grammatically, старых is in the prepositional plural, because в here means in.

Because в here means in, expressing location or position, and that normally takes the prepositional case.

So:

• в старых кроссовках = in the old sneakers

Compare:

• в доме = in the house
• в новой куртке = in the new jacket
• в старых кроссовках = in the old sneakers

If в meant movement into, then Russian would usually use the accusative instead. But here the idea is not motion into the shoes; it is the state of running while wearing them.

Уже means already, but in sentences like this it often has the nuance of by now, at this point, or now.

It suggests that the old sneakers have reached the point where running in them is no longer comfortable.

So the difference is roughly:

• в старых неудобно бегать = it’s uncomfortable to run in the old ones
• в старых уже неудобно бегать = it’s already uncomfortable to run in the old ones / by now it’s uncomfortable

In English, already may or may not be translated directly, but in Russian it adds a very natural nuance.

Here неудобно is not describing the sneakers directly. It is describing the situation: it is uncomfortable to run.

This is a very common Russian structure:

• неудобно + infinitive
• трудно + infinitive
• легко + infinitive
• приятно + infinitive

Examples:

• Мне трудно понять = It’s hard for me to understand
• Здесь удобно работать = It’s comfortable/convenient to work here
• В старых уже неудобно бегать = It’s already uncomfortable to run in the old ones

If you said неудобные кроссовки, that would mean uncomfortable sneakers, directly describing the shoes themselves.

Because бегать refers to running as a general activity, something repeated or done regularly, while бежать usually refers to running right now, often in one concrete direction.

Here the person means running in general, probably as exercise or routine use:

• неудобно бегать = it’s uncomfortable to run
• неудобно бежать would sound more like it’s uncomfortable to be running right now / to run at this moment

So бегать is the natural verb for general athletic activity.

Russian often leaves out мне when the person experiencing something is obvious from context.

Since the sentence begins with Я куплю..., it is already clear that the speaker is talking about themself. So:

• Я куплю кроссовки получше, потому что в старых уже неудобно бегать.

naturally implies:

• ...потому что мне в старых уже неудобно бегать.

Adding мне is possible, but not necessary. Without it, the sentence sounds natural and smooth.

Because потому что introduces a subordinate clause meaning because.

Russian normally separates that clause with a comma:

• Я куплю кроссовки получше, потому что...

This is standard punctuation in Russian, just as English often separates clauses in longer sentences, although Russian uses commas more consistently in these structures.