Sie stellt eine verblasste Bildunterschrift wieder her.

Here, Sie means she.

Why not they or formal you?

• they would take stellen
• formal you also takes stellen

But the sentence has stellt, which is 3rd person singular, so the subject must be she.

Because the sentence starts with Sie, it is capitalized anyway, so the verb form is what tells you the meaning.

Because wiederherstellen is a separable verb.

In a normal main clause, German often splits separable verbs like this:

• finite verb part in position 2: stellt
• prefix at the end: her

So:

• Sie stellt eine verblasste Bildunterschrift wieder her.

But in the infinitive or in some other structures, it stays together:

• wiederherstellen
• Sie will eine verblasste Bildunterschrift wiederherstellen.
• ..., weil sie eine verblasste Bildunterschrift wiederherstellt.

This is a very common German pattern called the sentence bracket.

With a separable verb in a main clause:

• the conjugated verb goes early, usually in second position
• the separable prefix goes at the end
• a lot of the rest of the sentence fits in the middle

So the structure is:

• Sie | stellt | eine verblasste Bildunterschrift | wieder her

That is normal German word order.

It is in the accusative case because it is the direct object: it is the thing being restored.

The verb wiederherstellen takes a direct object, so the thing restored is accusative.

Here:

• Sie = subject
• eine verblasste Bildunterschrift = direct object

With feminine nouns, eine looks the same in nominative and accusative, so the form itself does not change. You identify the case from the sentence function.

Because it comes after eine and before a feminine singular noun in the accusative.

German adjectives before nouns take different endings depending on:

• gender
• case
• article type

Here you have:

• eine = feminine singular
• Bildunterschrift = feminine
• accusative singular

So the adjective takes -e:

• eine verblasste Bildunterschrift

Compare:

• ein verblasstes Foto
• einen verblassten Text
• eine verblasste Notiz

In this sentence, it functions as an adjective.

It describes Bildunterschrift and means faded.

It comes from the verb verblassen, meaning to fade. The form verblasst can be used adjectivally, and here it gets the adjective ending -e:

• base adjective: verblasst
• before the noun: verblasste Bildunterschrift

So it is not the main verb of the sentence. The main verb is stellt ... her.

Because German very often builds nouns as compound nouns.

Bildunterschrift is made from:

• Bild = picture / image
• Unterschrift = signature / caption depending on context

Together, Bildunterschrift means caption, especially an image or photo caption.

A useful rule:

• the last part of a compound determines the gender and the basic noun type

So because die Unterschrift is feminine, die Bildunterschrift is also feminine.

It is the present tense.

German present tense can often correspond to different English translations depending on context, for example:

• she restores
• she is restoring

German does not require a separate progressive form the way English often does.

So the sentence is grammatically present tense, even if English might translate it in more than one way.

The dictionary form is wiederherstellen.

That is the infinitive.

Some useful forms are:

• infinitive: wiederherstellen
• present: sie stellt wieder her
• simple past: sie stellte wieder her
• perfect: sie hat wiederhergestellt

This is a good verb to learn as a full unit, not just as stellen + something extra.

In a subordinate clause, the separable verb is usually written as one word again and goes to the end:

• ..., weil sie eine verblasste Bildunterschrift wiederherstellt.

In the perfect tense, you use hat plus the past participle:

• Sie hat eine verblasste Bildunterschrift wiederhergestellt.

So the main-clause version is split:

• stellt ... her

But these forms are not:

• wiederherstellt
• wiederhergestellt

Sometimes, but the nuance changes.

wiederherstellen means something like:

• restore
• recover
• bring back to its earlier state

restaurieren is often used for things like:

• artworks
• furniture
• buildings
• historical objects

For a caption, wiederherstellen sounds very natural if the idea is that the text is being restored or recovered.

So restaurieren is not impossible in every context, but wiederherstellen is the more straightforward choice here.