В одном ботинке сбилась стелька, поэтому идти стало неудобно.

В одном ботинке is in the prepositional case.

• в
  • prepositional often means in / inside / within
• ботинок → ботинке in the singular prepositional
• один also changes to match: одном

So в одном ботинке literally means in one boot/shoe or in one of the shoes.

Here it describes the location where the problem happened: the insole got displaced in one shoe.

In Russian, один / одна / одно / одни can simply mean the number one, but it can also mean a certain one or one of several.

In this sentence, в одном ботинке naturally means in one of the shoes/boots. Since people normally wear a pair, Russian does not need to say one of the two shoes explicitly.

So the idea is:

• в ботинке = in the shoe
• в одном ботинке = in one shoe / in one of the shoes

Стелька means insole.

Here it is in the nominative singular, because it is the subject of the verb сбилась.

The structure is:

• стелька сбилась = the insole shifted / got bunched up / slipped out of place

Even though the sentence begins with в одном ботинке, that phrase is just a location phrase. The actual grammatical subject is стелька.

Here сбилась means something like:

• got displaced
• shifted out of place
• got bunched up
• slipped out of position

It comes from the verb сбиться.

In other contexts, сбиться can mean things like:

• to get confused
• to lose track
• to go off course
• to get knocked out of place

In this sentence, it is the physical meaning: the insole inside the shoe is no longer lying flat the way it should.

Because the subject стелька is feminine.

In the Russian past tense, the verb agrees with the subject in gender and number:

• masculine: сбился
• feminine: сбилась
• neuter: сбилось
• plural: сбились

Since стелька is a feminine noun, the correct form is сбилась.

The -сь / -ся ending often makes a verb reflexive, but that does not always mean a literal self-action. Very often it means that something happened to the subject or that the subject ended up in a certain state.

Compare:

• Я сбил стельку = I knocked the insole out of place
• Стелька сбилась = the insole got knocked out of place / shifted

So сбилась is natural because the sentence is focused on what happened to the insole, not on who caused it.

Поэтому means therefore, so, or that’s why.

It connects the first clause with the consequence:

• В одном ботинке сбилась стелька = the insole shifted in one shoe
• поэтому идти стало неудобно = so walking became uncomfortable

It is very common in everyday Russian for expressing cause and result.

Because the sentence contains two clauses:

1. В одном ботинке сбилась стелька
2. поэтому идти стало неудобно

Russian normally places a comma before поэтому when it links one clause to the result in the next clause.

So the comma works much like in English:

• The insole shifted in one shoe, so it became uncomfortable to walk.

This is a very common Russian structure:

• infinitive + стало + predicative adverb

Here:

• идти = to walk / to go on foot
• стало = became
• неудобно = uncomfortable / inconvenient

So идти стало неудобно literally means to walk became uncomfortable, but in natural English we would say:

• it became uncomfortable to walk
• walking became uncomfortable

Russian often uses the infinitive this way where English might use it + be + adjective + infinitive.

Because неудобно here is not an ordinary adjective modifying a noun. It is a predicative adverb/state word, used in impersonal constructions.

Russian often says:

• мне трудно
• ему холодно
• здесь удобно
• идти неудобно

These do not describe a noun directly. They describe a state or situation.

So:

• стало неудобно = it became uncomfortable

The verb стало is in the neuter singular, which is very common in impersonal sentences.

If you said неудобным, that would be instrumental singular of the adjective and would need a noun to agree with, such as:

• путь стал неудобным = the route became inconvenient

But that is a different structure.

Both verbs relate to going on foot, but they are not used in the same way.

• идти = to be going, to go in one direction, to walk at a particular time
• ходить = to go habitually, repeatedly, or in various directions

Here the sentence refers to the immediate physical act of walking in those shoes. So идти стало неудобно means walking became uncomfortable right then and there.

If you used ходить, it would sound more like a general or repeated situation:

• ходить стало неудобно = it became uncomfortable to walk around / to wear them and walk generally

That version is also possible in some contexts, but идти fits the immediate situation very well.

Yes. Russian word order is flexible, and different orders change the emphasis more than the basic meaning.

For example:

• В одном ботинке сбилась стелька
  This naturally sets the scene first: in one shoe...
• Стелька в одном ботинке сбилась
  This puts more attention on the insole
• Поэтому стало неудобно идти
  This is also correct and very natural
• Поэтому идти стало неудобно
  This slightly emphasizes the action to walk

So the original sentence is normal and natural, but other word orders are possible depending on what the speaker wants to highlight.

Ботинок usually refers to a sturdier shoe, often something like a boot or ankle boot, but in everyday speech it can sometimes be translated more loosely as shoe depending on context.

So in this sentence, English could use either:

• boot
• shoe

The important part is that it is footwear with an insole inside it.

It can mean different kinds of inconvenience or discomfort, depending on context.

For example:

• Мне неудобно сидеть = it’s uncomfortable for me to sit
• Мне неудобно просить = I feel awkward asking
• Нам неудобно ехать в это время = it’s inconvenient for us to go at that time

In this sentence, because the insole shifted inside the shoe, неудобно clearly means physically uncomfortable.

So идти стало неудобно means that walking became uncomfortable because of the shoe problem.