A vendég kezében van a kulcs.

Word by word:

• A – the (definite article, used before consonant)
• vendég – guest
• kezében – literally in his/her hand (more precisely: in the hand (that belongs to him/her))
  • kéz – hand
  • -e – 3rd‑person possessive suffix (his/her/its hand)
  • -ben – inessive case ending (in, inside)
• van – is (3rd person singular of lenni – to be)
• a – the
• kulcs – key

So the structure is something like:
The guest hand-in is the key. → The key is in the guest’s hand.

Hungarian normally uses case endings instead of separate prepositions for many spatial relations.

• English: in the hand
• Hungarian: the noun kéz
  • inessive suffix -ben → kézben = in (a) hand

In our sentence, the hand also has an owner (the guest), so we add a possessive suffix:

• kéz (hand)
• • -e (3rd person possessive: his/her hand) → keze
• • -ben (in) → kezében = in his/her hand

So kezében packs “in his/her hand” into one word:

• kéz
  • -e
    • -ben = hand
      • his/her
        • in

The base form is kéz (with a long é), but when you add possessive endings, the stem usually changes from kéz- to kez-:

• kéz – hand
• keze – his/her hand
• kezében – in his/her hand

So the vowel shortens (é → e) in the possessed forms. This is a regular stem change for this word, not a spelling mistake. You will see this pattern in some other Hungarian words as well, where the stem slightly changes when suffixes are added.

-ben is the inessive case ending. It expresses something like:

• in, inside, within

Examples:

• ház – house → házban – in the house
• táska – bag → táskában – in the bag
• kéz – hand → kézben – in (a) hand
• kezében – in his/her hand

The choice -ban vs -ben follows vowel harmony:

• back vowels → -ban
• front vowels → -ben

kéz / keze has front vowels, so it takes -ben → kezében.

In Hungarian, possession is shown by a suffix on the possessed noun, not by a separate word like his/her.

For kéz (hand):

• kezem – my hand
• kezed – your hand (singular)
• keze – his/her hand
• kezünk – our hand(s)
• kezetek – your hand(s) (plural)
• kezük – their hand(s)

In our sentence:

• kezé- – his/her hand (3rd person possessive)
• -ben – in

So:

• kezében = in his/her hand

The “his/her” idea is included in the -e part of keze-.

The guest is the possessor of the hand, expressed as a separate noun phrase before the possessed noun:

• a vendég keze – the guest’s hand
• a vendég kezében – in the guest’s hand

Structure:

• a vendég – the guest (possessor, in nominative)
• kezében – in his/her hand (possessed noun with possessive + case)

Hungarian often shows the possessor like this:
[possessor in plain form] + [possessed noun with possessive suffix]

So:

• a vendég kezében literally: in the hand of the guest → in the guest’s hand

Both are possible, but they are used a bit differently:

1. Simple possessor (no -nak/‑nek)

   • a vendég keze – the guest’s hand
   • a vendég kezében – in the guest’s hand
     This is very common and neutral.

2. Dative possessor (with -nak/‑nek)

   • a vendégnek a keze – the hand of the guest
   • a vendégnek a kezében – in the hand of the guest
     This often adds a bit of emphasis on the possessor (the guest), or appears in certain syntactic structures.

In your sentence, A vendég kezében van a kulcs is perfectly natural and standard.
A vendégnek a kezében van a kulcs is also grammatically fine, but it feels slightly more emphatic or contrastive (e.g. “It’s in the guest’s hand (not someone else’s)”).

Hungarian can drop van (is) in some situations, but not in all.

You can omit van when:

• the predicate is a noun or adjective, not a location, and
• it’s in the present tense, 3rd person singular:

Examples:

• Ő orvos. – He/She is a doctor. (no van)
• A kulcs új. – The key is new. (no van)

But you must keep van when:

• the predicate expresses location or existence (where something is), as in your sentence:

• A kulcs a vendég kezében van. – The key is in the guest’s hand.
• A kulcs az asztalon van. – The key is on the table.

So here van cannot be omitted; it’s required with this kind of “in/on/under/at” location.

Yes, that word order is also correct:

• A vendég kezében van a kulcs.
• A kulcs a vendég kezében van.

Hungarian word order is used to show focus and what is new or important:

• A kulcs a vendég kezében van.
  Neutral way if you’re simply stating where the key is. The topic is “the key”, and you tell something about it: it is in the guest’s hand.

• A vendég kezében van a kulcs.
  This tends to emphasize the location (“in the guest’s hand”) more strongly, as if contrasting it with other possible places. For example, in a context like:

  • “Is the key on the table?” – No, a vendég kezében van a kulcs.

Both sentences are grammatical; the choice depends on what you’re highlighting: the key or its location.

Yes, you can say:

• A vendég kezében a kulcs van.

In this order, a kulcs (the key) is in the focus position just before van. This typically means:

• It’s the key that is in the guest’s hand (and not something else).

So the nuances:

• A vendég kezében van a kulcs. – The key is in the guest’s hand. (emphasis on the location)
• A vendég kezében a kulcs van. – It’s the key that’s in the guest’s hand (not the wallet, not the phone, etc.)

Hungarian uses word order to mark this kind of focus meaning very systematically.

Hungarian normally uses the definite article a / az much like English “the”:

• a vendég – the guest
• a kulcs – the key

In A vendég kezében van a kulcs, both nouns are specific:

• it’s a particular guest
• and a particular key

So both take the article. That’s completely normal in Hungarian.

You might see different article behavior with possessed nouns, but here:

• the possessor: a vendég (with article)
• the possessed: kezében (no separate article inside the word)
• the subject: a kulcs (with article)

With possessed nouns, when the possessor is an explicit noun (like the guest), Hungarian usually does not add a separate article to the possessed noun in this structure:

• a vendég keze – the guest’s hand (not a vendég a keze)
• a vendég kezében – in the guest’s hand

Compare:

• a keze – his/her hand (with article, but possessor is only implied by the possessive suffix)
• a vendég keze – the guest’s hand (no extra article before keze)

So:

• a vendég kezében is the natural form.
• a vendég a kezében would be ungrammatical in this meaning.

Hungarian nouns have no grammatical gender.

• vendég simply means “guest”, without specifying male or female.
• Likewise, the possessive suffix -e in keze / kezében does not show gender:
  • keze = his hand or her hand or its hand depending on context.

So A vendég kezében van a kulcs is gender‑neutral:
“The key is in the guest’s hand.” (no indication of male/female).

Hungarian often uses singular where English would use plural, especially with body parts:

• Megfogta a kezem. – He/She took my hand. (not necessarily one hand only)
• Fáj a hátam. – My back hurts. (not backs)

Similarly:

• a vendég kezében – literally: in the guest’s hand
  → in natural English, this could be understood as either in the guest’s hand or in the guest’s hands, depending on context.

If you really want to stress more than one hand, you can use a plural form:

• a vendég kezeiben – in the guest’s hands but that’s less common; kezében is usually enough.

You need to pluralize both vendég and kulcs, and use the plural form of van:

• A vendégek kezében vannak a kulcsok.

Breakdown:

• A vendégek – the guests (plural of vendég)
• kezében – in their hand(s) (Hungarian often still uses the singular possessed noun)
• vannak – are (3rd person plural of lenni)
• a kulcsok – the keys

So:

• A vendég kezében van a kulcs. – The key is in the guest’s hand.
• A vendégek kezében vannak a kulcsok. – The keys are in the guests’ hands.