Die gestohlene Wertsache fehlt noch.

Die is the definite article, meaning the.

In this sentence, it tells you that Wertsache is:

• feminine
• singular
• nominative here, because it is the subject of the verb fehlt

So die gestohlene Wertsache = the stolen valuable item / the stolen valuable.

Because gestohlen is being used like an adjective in front of the noun Wertsache.

In German, adjectives placed before nouns usually take an ending. Here:

• die = feminine singular definite article
• Wertsache = feminine singular noun
• so the adjective gets the ending -e

That gives:

• die gestohlene Wertsache

Compare:

• die gestohlene Wertsache = the stolen valuable item
• Die Wertsache ist gestohlen. = The valuable item is stolen.

In the second sentence, gestohlen comes after the verb and does not take an ending.

It comes from a verb form.

Gestohlen is the past participle of stehlen = to steal.
Here that participle is being used adjectivally, meaning stolen.

This is very common in German, just like in English:

• the stolen bike
• das gestohlene Fahrrad

So it is originally a verb form, but in this sentence it functions like an adjective modifying Wertsache.

Wertsache means valuable item, valuable, or valuables depending on context.

It is a compound noun:

• Wert = value
• Sache = thing, item

So literally it is something like value-thing, meaning an item of value.

It is a feminine noun, which is why the article is die.

Singular:

• die Wertsache = the valuable item

Plural:

• die Wertsachen = the valuables / valuable items

Because the subject is singular:

• die gestohlene Wertsache

Since that whole noun phrase is singular, the verb must also be singular:

• Die gestohlene Wertsache fehlt noch.

If it were plural, you would say:

• Die gestohlenen Wertsachen fehlen noch.

So:

• fehlt = singular
• fehlen = plural

Here fehlen means to be missing.

That is an important point, because German fehlen does not work exactly like English to miss.

In this sentence:

• Die gestohlene Wertsache fehlt noch.
• literally: The stolen valuable item is still missing.

The thing itself is the subject.

English often says:

• The stolen item is still missing.

German uses fehlen naturally for that idea.

Be careful not to confuse it with English I miss something. In German, that idea is usually expressed differently, for example:

• Du fehlst mir. = I miss you.
  Literally: You are missing to me.

So in your sentence, fehlt means is missing, not misses.

Here noch means still.

So:

• Die gestohlene Wertsache fehlt noch.
• = The stolen valuable item is still missing.

It adds the idea that the situation continues up to now.

Depending on context, noch can have other meanings too, such as yet, another, or more, but here still is the natural meaning.

This is a normal German main clause.

German main clauses follow the verb-second rule, often called V2:

• one element comes first
• the finite verb comes second

Here the first element is the subject:

• Die gestohlene Wertsache | fehlt | noch

That is the most neutral word order.

You could also move another element to the first position for emphasis:

• Noch fehlt die gestohlene Wertsache.

That sounds more like:

• The stolen valuable item is still missing.

Same basic meaning, but slightly different focus.

Not normally, no.

Gestohlen means stolen, but ist gestohlen focuses on the fact that it was stolen, not on the fact that it is still missing.

• Die Wertsache ist gestohlen. = The valuable item has been stolen / is stolen.
• Die Wertsache fehlt noch. = The valuable item is still missing.

Your original sentence combines both ideas by describing the item as the stolen valuable item and then saying it is still missing.

So fehlt noch is the natural choice if the important idea is that the item has not been recovered.

In the plural, several parts of the sentence change:

Singular:

• Die gestohlene Wertsache fehlt noch.

Plural:

• Die gestohlenen Wertsachen fehlen noch.

Changes:

• gestohlene → gestohlenen
• Wertsache → Wertsachen
• fehlt → fehlen

This is a useful pattern to remember:

• die gestohlene Wertsache
• die gestohlenen Wertsachen

Yes.

The finite verb is fehlt, which is present tense:

• fehlt = is missing

The word gestohlen comes from a past participle, but here it is part of the description of the noun:

• the stolen valuable item

So the sentence means:

• the item was stolen earlier
• and it is still missing now

That is why the sentence combines a past event with a present situation very naturally.