Wir behalten den Fotoabzug, es sei denn, er ist stark vergilbt.

Es sei denn means unless or except if. It introduces an exception to what was just said.

In this sentence:

Wir behalten den Fotoabzug, es sei denn, er ist stark vergilbt.

the basic statement is:

Wir behalten den Fotoabzug = We keep the photo print

Then es sei denn adds the exception:

unless it is badly yellowed

So the pattern is often:

Statement + es sei denn + exception

It is a very common fixed expression in German.

This is one of the first things learners notice. Sei is the Konjunktiv I form of sein. In es sei denn, German uses this form as part of a fixed expression.

So although sei may look unusual, in modern German es sei denn is best learned as a whole chunk meaning unless.

Important point:

• Inside the fixed phrase, it is sei
• After that, the following clause can still use normal indicative verb forms, as in: es sei denn, er ist stark vergilbt

So you do not need to think of the whole sentence as being in a special mood. Just remember:

• es sei denn = fixed expression
• the rest of the clause is often normal German

Because Fotoabzug is the direct object of behalten.

The verb behalten means to keep, and what is being kept is the object:

• Wir behalten was?
  den Fotoabzug

Fotoabzug is a masculine noun:

• nominative: der Fotoabzug
• accusative: den Fotoabzug

So:

• Der Fotoabzug ist neu. = nominative, subject
• Wir behalten den Fotoabzug. = accusative, direct object

Because Fotoabzug is a masculine noun, so the matching pronoun is er.

• der Fotoabzug → er

In the sentence:

..., es sei denn, er ist stark vergilbt.

er means the photo print.

Even though in English we might say it, German pronouns usually follow the grammatical gender of the noun:

• masculine noun → er
• feminine noun → sie
• neuter noun → es

A Fotoabzug is a photo print: a physical printed photograph, not just a digital image or the abstract idea of a photo.

That matters here because a print can become vergilbt over time, meaning it turns yellow with age.

So Fotoabzug is more specific than just Foto in some contexts.

Vergilbt means yellowed or discolored so that it has become yellow, usually because of age, sunlight, bad storage, or paper quality.

It comes from the verb vergilben, meaning to turn yellow.

In this sentence:

• er ist vergilbt = it is yellowed
• stark vergilbt = badly / heavily yellowed

So this describes the condition of the photo print.

Stark is an adverb here, meaning strongly, badly, or heavily.

So:

• vergilbt = yellowed
• stark vergilbt = severely yellowed / badly yellowed

It intensifies the adjective-like participle vergilbt.

Other similar examples:

• stark beschädigt = badly damaged
• stark verschmutzt = very dirty / heavily soiled

Because after es sei denn, German often uses normal main-clause word order.

So you get:

• es sei denn, er ist stark vergilbt

with the finite verb ist in second position in the clause.

Learners sometimes expect subordinate-clause word order everywhere after an expression meaning unless, but es sei denn does not behave exactly like a standard subordinating conjunction such as weil or dass.

You may also see:

• es sei denn, dass er stark vergilbt ist

That version uses dass, so the verb goes to the end:

• ... dass er stark vergilbt ist

But the shorter version without dass is very common.

There is a comma because es sei denn introduces an additional clause or clause-like exception.

German punctuation often separates such parts clearly:

• main statement
• exception

So: Wir behalten den Fotoabzug, es sei denn, er ist stark vergilbt.

The comma helps show:

1. the main statement: We keep the print
2. the exception: unless it is badly yellowed

Sometimes wenn nicht can express a similar idea, but it is not always as natural or as precise as es sei denn.

For this sentence, es sei denn is the most idiomatic choice for unless.

Compare:

• Wir behalten den Fotoabzug, es sei denn, er ist stark vergilbt.
  = natural, clear exception

A version with wenn nicht would sound more like:

• Wir behalten den Fotoabzug, wenn er nicht stark vergilbt ist.

That means:

• We keep the photo print if it is not badly yellowed

This is close in meaning, but the structure is different:

• es sei denn = state the rule, then add the exception
• wenn nicht = give a condition directly

In this sentence, behalten means to keep or to retain.

That is the most important meaning here:

• Wir behalten den Fotoabzug = We keep the photo print

But behalten can also be used in other contexts, such as:

• Darf ich das Buch behalten? = May I keep the book?
• Ich behalte das im Kopf. = I’ll keep that in mind
• Sie behielt recht. = less common/literary uses depending on context

For this sentence, the simple meaning keep is exactly right.

It is in the present tense.

• Wir behalten = present tense of behalten
• er ist = present tense of sein

German often uses the present tense for general rules, habits, or decisions, just as English does:

• We keep the photo print unless it is badly yellowed.

So this sentence sounds like a general policy or standard practice.