Ten řetěz, co leží u kola, potřebuje mazivo.

Not exactly. Ten literally means that, but in everyday Czech it is often used a bit like English the/that, especially when pointing out a specific thing.

So ten řetěz means something like:

• that chain
• or, in context, the chain

A few notes:

• řetěz is a masculine inanimate noun.
• In the nominative singular, the demonstrative is ten for masculine inanimate nouns.

You could often leave ten out and just say Řetěz, co leží u kola, potřebuje mazivo, but adding ten makes it more specific or noticeable.

Řetěz is in the nominative singular.

Why?

Because it is the subject of the main verb potřebuje (needs).

Structure:

• Ten řetěz = the subject
• co leží u kola = a relative clause describing the chain
• potřebuje mazivo = the main statement about the chain

So the chain is the thing that needs the lubricant.

Here co means that/which in a relative clause.

So:

• řetěz, co leží u kola = the chain that is lying near the bike / wheel

This use of co is very common in everyday spoken Czech.

A more formal or standard version would be:

• Ten řetěz, který leží u kola, potřebuje mazivo.

So:

• co = common, informal, conversational
• který = more formal, more clearly standard written Czech

Both are understood, but co feels more natural in casual speech.

Because co leží u kola is a relative clause inserted into the sentence.

It gives extra information about which chain we are talking about:

• Ten řetěz, co leží u kola, potřebuje mazivo.

The commas mark off that descriptive clause, just like in English writing you might separate inserted information.

In Czech, punctuation around relative clauses is important and used more consistently than in English.

Leží comes from ležet, which means:

• to lie
• to be lying
• sometimes more generally to be situated / to be located

So co leží u kola literally means:

• that is lying near the bicycle/wheel

Czech often uses verbs like ležet, stát, viset, etc. to describe position:

• kniha leží na stole = the book is lying on the table
• láhev stojí na stole = the bottle is standing on the table
• obraz visí na zdi = the picture is hanging on the wall

Using je u kola would just mean is near the bike/wheel, which is possible, but leží gives a more physical picture of its position.

Kola is in the genitive singular because the preposition u requires the genitive.

So:

• u
  • genitive = by / near / at

Examples:

• u domu = by the house
• u stolu = by the table
• u kola = by the bicycle / by the wheel

The noun kolo has nominative singular kolo, but after u it becomes kola.

It can mean either, depending on context.

Kolo can mean:

• wheel
• bicycle

In this sentence, both are grammatically possible:

• the chain lying by the wheel
• the chain lying by the bicycle

In real life, many learners would assume bicycle because a chain is strongly associated with a bike, but the exact meaning depends on the wider context.

Because mazivo is the direct object, and here it is in the accusative singular.

For the neuter noun mazivo, the nominative and accusative singular have the same form:

• nominative: mazivo
• accusative: mazivo

So:

• řetěz potřebuje mazivo = the chain needs lubricant

The form maziva would usually be a different case/number, not the normal direct object form here.

Mazivo is a general word meaning lubricant.

Depending on context, in English you might translate it as:

• lubricant
• oil
• grease

But mazivo itself is broader than just one specific product. It simply means a substance used for lubrication.

So the Czech sentence does not necessarily specify whether it is oil or grease, only that the chain needs something to lubricate it.

Potřebuje is the 3rd person singular present tense of potřebovat (to need).

Conjugation:

• já potřebuji / potřebuju = I need
• ty potřebuješ = you need
• on/ona/ono potřebuje = he/she/it needs

Since the subject is ten řetěz (that chain), which is singular, the verb must also be singular:

• Ten řetěz potřebuje... = That chain needs...

Yes.

You can say:

• Ten řetěz, co leží u kola, potřebuje mazivo.
• Ten řetěz, který leží u kola, potřebuje mazivo.

The difference is mostly style:

• co = more colloquial, very common in speech
• který = more formal, more standard in careful writing

For many learners, který is taught first because it behaves more like a textbook relative pronoun. But in real spoken Czech, co is extremely common.

Yes. Czech word order is flexible, although not completely free.

The neutral version here is:

• Ten řetěz, co leží u kola, potřebuje mazivo.

But you might also hear variations depending on emphasis, for example:

• Mazivo potřebuje ten řetěz, co leží u kola.

That version emphasizes mazivo first, as if answering What does that chain need?

Even though the order can change, the cases help show who is doing what, so the meaning usually stays clear.

Yes, it is natural, especially in everyday speech.

A few style notes:

• Ten řetěz sounds conversational and specific.
• co as a relative pronoun is very common in spoken Czech.
• potřebuje mazivo is perfectly natural.

If you wanted a more formal or written style, you might say:

• Ten řetěz, který leží u kola, potřebuje mazivo.

So the original sentence sounds natural and ordinary, especially in speech.