Onko teillä kestävä varaosa tähän vanhaan pyörään?

Onko is the verb on (is) plus the yes/no question marker -ko/-kö.

So:

• on = is
• onko = is ... ? / is there ... ? / do you have ... ?

In Finnish, a neutral yes/no question usually puts the finite verb first:

• Onko teillä ... ? = Do you have ... ?

So this is the normal way to begin a question like this.

Yes. Teillä is related to te, which means you in the plural, and it is also commonly used as a polite you when speaking to one person in a service situation.

In a shop, Onko teillä ... ? is very natural because it can mean:

• Do you have ... ?
• Does your shop have ... ?

So even if you are speaking to one employee, teillä sounds polite and normal.

Finnish usually does not use a separate verb like English have for possession. Instead, it uses olla (to be) plus a person in the adessive case.

So the pattern is:

• Minulla on = I have
• Sinulla on = you have
• Teillä on = you have / you all have / your shop has

Literally, Onko teillä varaosa? is something like:

• Is there a spare part with you / at your place?

That is the normal Finnish way to express possession.

Here kestävä means durable, long-lasting, or sturdy.

It comes from the verb kestää, which can mean to last, to endure, or to withstand.

So:

• kestävä varaosa = a durable spare part

Be aware that kestävä can also mean sustainable in other contexts. For example:

• kestävä kehitys = sustainable development

In this sentence, though, durable is the more natural meaning.

Varaosa is in the singular nominative, which is the basic dictionary form.

That is normal here because the sentence is asking whether there exists a spare part, meaning one whole item. In Finnish existential and possession-type sentences, a singular countable thing is often in the nominative:

• Onko teillä varaosa? = Do you have a spare part?

A useful contrast is the negative:

• Teillä ei ole varaosaa. = You do not have a spare part.

In negative sentences, Finnish often uses the partitive, so varaosa becomes varaosaa.

That whole phrase is in the illative case, which often means into, to, or in context for.

So:

• tähän = into/to this
• vanhaan = into/to old
• pyörään = into/to bike

In English, we would usually translate the whole phrase more naturally as:

• for this old bike

With words like osa or varaosa, Finnish often uses the illative to show what the part is meant for or fits into.

So varaosa tähän vanhaan pyörään means a spare part intended for that bike.

Because Finnish adjectives agree with the nouns they describe in case and number.

So:

• vanha pyörä = old bike
• vanhaan pyörään = into/to/for the old bike

Both words change because the whole phrase is in the illative.

The same thing happens elsewhere in the sentence:

• kestävä varaosa = durable spare part

Both kestävä and varaosa are singular nominative there.

This kind of agreement is a very important feature of Finnish grammar.

Yes. Tähän is a case form of tämä (this).

Some common forms are:

• tämä = this
• tämän = this / of this
• tässä = in this
• tästä = from this
• tähän = into/to this

So tähän pyörään means to/for this bike.

Demonstratives like tämä have forms that may look a bit irregular at first, so they are worth memorizing as a set.

Yes, it is polite and natural.

Onko teillä ... ? is a very common way to ask staff in a shop whether they have something.

The sentence sounds reasonably careful and polite, especially because of teillä.

Some nearby alternatives are:

• Onko teillä varaosaa tähän vanhaan pyörään? = Do you have a spare part / any spare parts for this old bike?
• Löytyykö teiltä varaosaa tähän vanhaan pyörään? = Would you happen to have a spare part for this old bike?

The original sentence is perfectly understandable and natural, especially if you want to emphasize that the spare part should be durable.

Yes, Finnish word order is fairly flexible, because the case endings show how words relate to each other.

The original order is clear and natural:

• Onko teillä kestävä varaosa tähän vanhaan pyörään?

But you could also move parts around to change emphasis, for example:

• Onko teillä tähän vanhaan pyörään kestävä varaosa?

That version puts a bit more focus on this old bike.

So the word order can change, but the original sentence is a good neutral model for learners.