A kulcstartón van a lakás kulcsa is, ezért mindig nálam marad.

Kulcstartó means keychain / keyring.

The ending -n is a case ending, specifically the superessive case, which often means on or on top of something.

So:

• kulcstartó = keychain
• kulcstartón = on the keychain

In this sentence, A kulcstartón van... means It is on the keychain...

Hungarian often uses this on idea for things attached to a surface or hanging from something, so on the keychain is natural here.

Because Hungarian shows possession differently from English.

In a lakás kulcsa:

• a lakás = the apartment
• kulcsa = its key / the key of it

Hungarian usually marks the possessed thing with a possessive ending.

So:

• kulcs = key
• kulcsa = his/her/its key

That is why a lakás kulcsa literally means something like the apartment its key, but in natural English it is the apartment key or the key to the apartment.

This is normal in Hungarian possessive structures when both parts are definite.

So:

• a lakás = the apartment
• a lakás kulcsa = the apartment's key / the apartment key

Hungarian often keeps the article with the possessor:

• a ház ablaka = the house's window
• a nő táskája = the woman's bag
• a lakás kulcsa = the apartment key

English usually does not repeat the in the same way, but Hungarian does.

Is means also / too / as well.

In this sentence:

• a lakás kulcsa is = the apartment key too / also the apartment key

A very important point: is usually comes after the word or phrase it belongs to.

Examples:

• én is = me too / I also
• itt is = here too
• a kulcs is = the key too

So here, kulcsa is tells you that the apartment key is also on the keychain.

Van is the verb to be.

Hungarian often leaves out van in the present tense when talking about identity or description:

• A ház nagy. = The house is big.
• Ő orvos. = He is a doctor.

But when talking about location or existence, Hungarian normally uses van:

• A könyv az asztalon van. = The book is on the table.
• A kulcstartón van... = It is on the keychain...

So in this sentence, van is needed because the sentence is about where something is.

Hungarian word order is flexible, and it often shows topic and focus, not just basic grammar.

This version starts with A kulcstartón:

• A kulcstartón = as for the keychain / on the keychain
• then van
• then a lakás kulcsa is

So the sentence is presenting the keychain first, almost like:

• On the keychain, there is also the apartment key.

A different order is also possible, for example:

• A lakás kulcsa is a kulcstartón van.

That would put stronger emphasis on the apartment key too.

So the given sentence is grammatical; it just organizes the information in a particular way.

Ezért means therefore, because of this, or for this reason.

It connects the two ideas:

• the apartment key is also on the keychain
• therefore it always stays with me

So:

• ezért = therefore / that’s why

It is a very common connector in Hungarian.

Nálam means at me or, more naturally in English, with me / in my possession.

It comes from:

• -nál / -nél = at, by, with someone
• nálam = at me / with me
• nálad = with you
• nála = with him/her

Examples:

• Nálam van a kulcs. = I have the key. / The key is with me.
• Nála maradt a könyv. = The book stayed with him/her.

In this sentence, mindig nálam marad means it always stays with me.

This is a very useful distinction.

• velem = with me in the sense of together with me
• nálam = with me / in my possession / at my place

Here the meaning is about the key being in your possession, not about it accompanying you as a companion. So nálam is the natural choice.

Compare:

• A kulcs nálam van. = The key is with me.
• Gyere velem! = Come with me!

So for objects that someone has, nálam is often the right form.

Marad means stays / remains.

So:

• mindig nálam marad = it always stays with me

This is slightly different from just saying van.

Compare:

• nálam van = it is with me
• nálam marad = it stays/remains with me

The verb marad highlights the result: because the apartment key is on the keychain, it does not get left behind somewhere else; it remains with me.

The subject is understood from context, so Hungarian does not need to say it explicitly.

In English, you would say:

• so it always stays with me

In Hungarian, the pronoun ő/az is often omitted when it is clear from context.

Here, the understood subject is the thing being talked about in the previous clause, most naturally the apartment key.

This is very common in Hungarian: once the topic is clear, the language often leaves the subject unstated.