Unde sunt cheile mele?

Because cheile means the keys, which is plural. In Romanian, the verb a fi (to be) changes according to number:

• este / e = is for singular
• sunt = are for plural

So:

• Unde este cheia mea? = Where is my key?
• Unde sunt cheile mele? = Where are my keys?

Cheie means key in the singular.
Its plural is chei = keys.

Romanian usually adds the definite article to the end of the noun, instead of putting a separate word like the in front.

So:

• chei = keys
• cheile = the keys

This ending -le is the definite article here.

Because mele agrees with a plural feminine noun.

Romanian possessive words change depending on the gender and number of the thing possessed:

• mea = my (feminine singular)
• mele = my (feminine plural)

Since chei / cheile is feminine plural, you say:

• cheia mea = my key
• cheile mele = my keys

You usually learn the noun together with its gender. In this case, cheie is a feminine noun.

You can see it from forms like:

• o cheie = a key
• doua chei = two keys
• cheia mea = my key

The possessive mea / mele also shows that the noun is feminine.

This is the normal and natural Romanian word order for a simple question:

• Unde = where
• sunt = are
• cheile mele = my keys

So literally: Where are my keys?

Romanian often uses this same pattern in questions with unde:

• Unde este cartea? = Where is the book?
• Unde sunt copiii? = Where are the children?

You could sometimes change word order for emphasis, but Unde sunt cheile mele? is the standard form.

Both are correct, but they mean slightly different things:

• Unde sunt cheile? = Where are the keys?
• Unde sunt cheile mele? = Where are my keys?

If it is already obvious whose keys you mean, Romanian speakers may sometimes leave out the possessive. But if you specifically want to say my keys, keep mele.

A helpful approximation is:

• cheile ≈ KAY-eh-le

More carefully:

• chei sounds roughly like English kay
• -le sounds like leh

So cheile is about KAY-leh, with a slight extra vowel transition: KAY-eh-leh.

Also:

• ch before e in Romanian sounds like k, not like English ch in chair

So cheie does not sound like English chewy or chay with a ch sound.

Yes, sunt is the normal present plural form of to be for we/you plural/they in standard Romanian.

Examples:

• Noi suntem acasa. = We are at home.
• Voi sunteti gata. = You are ready.
• Ele sunt aici. = They are here.

In your sentence, sunt means are because the subject is plural: cheile mele.

Yes. Unde-s cheile mele? is a common shortened spoken form of Unde sunt cheile mele?

Here:

• unde-s = where are

This is natural in conversation, but in learning materials and careful writing, you will usually first see the full form Unde sunt cheile mele?

Because Romanian usually uses a postposed definite article, meaning the definite article is attached to the end of the noun.

Compare English and Romanian:

• the key → cheia
• the keys → cheile

So instead of saying a separate word like the, Romanian modifies the noun itself.

That is why cheile mele literally looks something like keys-the my, even though the natural English translation is my keys.

Yes. Unde just means where and does not change for singular or plural.

For example:

• Unde este telefonul? = Where is the phone?
• Unde sunt cheile? = Where are the keys?

The part that changes is the verb:

• este for singular
• sunt for plural

The singular version is:

Unde este cheia mea?

Breakdown:

• Unde = where
• este = is
• cheia = the key
• mea = my

So:

• Unde este cheia mea? = Where is my key?
• Unde sunt cheile mele? = Where are my keys?