Jos reikä kasvaa vielä isommaksi, suutari vaihtaa kenkään myös uuden pohjallisen.

Jos means if. It introduces a condition:

Jos reikä kasvaa vielä isommaksi
= If the hole gets even bigger

In Finnish, the jos clause works very much like an English if clause. When it comes first, it is usually followed by a comma, and then the main clause comes after it.

This is very normal Finnish.

Finnish often uses the present tense to talk about the future, especially in sentences like this:

Jos X tapahtuu, Y tapahtuu.
= If X happens, Y will happen.

So:

• kasvaa = literally grows / gets bigger
• vaihtaa = literally changes / replaces

But in context, the whole sentence can naturally refer to a future result:

If the hole gets even bigger, the cobbler will also replace the insole.

Finnish usually does not need a separate future tense here.

Because kasvaa here means grow/become, and in that pattern Finnish often uses the translative case ending -ksi to show the new state.

So:

• isompi = bigger
• isommaksi = into a bigger state / to become bigger

That is why:

reikä kasvaa isommaksi
means
the hole gets bigger

A useful contrast:

• Reikä on isompi. = The hole is bigger.
• Reikä kasvaa isommaksi. = The hole is getting/becoming bigger.

That comes from the comparative form of iso.

The pattern is:

• iso = big
• isompi = bigger
• stem used in inflected forms: isomma-
• isommaksi = into a bigger state

So isommaksi is just the correctly inflected form of the comparative isompi.

This is a normal pattern with comparatives in Finnish.

Here vielä means something like still, yet, or even depending on how you translate it.

In this sentence:

vielä isommaksi

it gives the sense of:

• even bigger
• bigger still

So it strengthens the comparison. The idea is not just bigger, but bigger than it already is now.

Because reikä is the subject of kasvaa.

In Finnish, the subject is very often in the nominative case, which is the basic form of the noun. So:

• reikä = hole
• it is the thing that is growing/getting bigger

That is why no extra case ending is needed there.

Kenkään is the illative singular of kenkä.

The illative case often means into something.

So:

• kenkä = shoe
• kenkään = into the shoe

That fits the meaning here, because the cobbler is putting/replacing something into the shoe.

A useful comparison:

• kengässä = in the shoe (location, already there)
• kenkään = into the shoe (movement/change toward the inside)

So the sentence is not just describing where something is; it is describing an action directed into the shoe.

Because this is the object of the verb vaihtaa.

With many Finnish verbs, when the action affects the whole object as a complete result, Finnish uses the total object. In the singular, that form usually looks like the genitive:

• uusi pohjallinen = a new insole (basic form)
• uuden pohjallisen = a/the new insole as a complete object

So here the cobbler is replacing the whole insole, not just doing some incomplete action to it.

Also notice that the adjective agrees with the noun:

• uuden
  • pohjallisen

Both are in the same form.

For a learner, the simplest answer is:

It is the object form used for a total object, and in the singular that form usually looks the same as the genitive.

So in practice:

• form: uuden pohjallisen
• function: object of vaihtaa

Many grammar explanations will call this the accusative object form, but it is completely normal and useful to notice that it looks identical to the genitive in sentences like this.

Here vaihtaa means replace, change, or put in a new one.

So:

suutari vaihtaa kenkään myös uuden pohjallisen

means that the cobbler will also replace the insole in the shoe / put a new insole into the shoe as well.

This is a good example of how Finnish vaihtaa often covers meanings that English might express with different verbs such as:

• change
• replace
• swap
• put in a new one

The exact English translation depends on context.

Because myös usually comes near the part it is focusing on.

Here it focuses on uuden pohjallisen, so the meaning is:

the cobbler will also replace a new insole / the insole too

In other words, replacing the insole is an additional thing the cobbler will do.

If you move myös, the emphasis changes. For example:

• Myös suutari vaihtaa... = The cobbler also replaces...
  (someone else does too)
• suutari vaihtaa myös uuden pohjallisen = the cobbler replaces the insole too

So the position of myös matters for emphasis.

Because Finnish has no articles.

Unlike English, Finnish does not normally use separate words for a/an or the. Whether something is understood as definite or indefinite comes from:

• context
• word order
• the situation
• sometimes the case or the wider sentence

So:

• reikä can mean a hole or the hole
• suutari can mean a cobbler or the cobbler
• uuden pohjallisen can mean a new insole or the new insole

The exact English article depends on context, not on a separate Finnish word.