Wir nutzen Cloudspeicher, es sei denn, das Dateiformat passt nicht.

Yes—es sei denn is a standard way to express unless in German.

In this sentence:

• Wir nutzen Cloudspeicher, es sei denn, das Dateiformat passt nicht.

the second part gives the exception to the first part.

A very close English structure is:

• We use cloud storage, unless the file format doesn’t work / isn’t suitable.

It is a fixed expression, so it is best learned as one unit: es sei denn = unless / except if.

This is one of the most common learner questions. In es sei denn, the word sei is the Konjunktiv I form of sein.

But in practice, you usually do not need to analyze it every time. Es sei denn is mostly treated as a fixed expression. Native speakers do not usually think of it as a full productive subjunctive pattern here—they simply recognize it as the normal phrase meaning unless.

So:

• es sei denn = idiomatic fixed phrase
• not something you normally replace with es ist denn

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

Here we get:

• das Dateiformat passt nicht

That is standard verb-second word order:

• subject: das Dateiformat
• verb: passt
• negation: nicht

So even though es sei denn translates like unless, the clause after it often behaves more like an independent clause in terms of word order.

That is why you do not get:

• ... es sei denn, dass das Dateiformat nicht passt only?
  Actually, with dass, you can sometimes rephrase things, but the original structure with normal word order is the common direct form.

Because es sei denn introduces an exception clause, and German punctuation separates that clause with commas.

So in:

• Wir nutzen Cloudspeicher, es sei denn, das Dateiformat passt nicht.

the commas mark off the inserted exception phrase.

You can think of it like this:

• main statement: Wir nutzen Cloudspeicher
• exception: es sei denn, das Dateiformat passt nicht

German commas are generally used more systematically than English commas with clause structures like this.

Because Cloudspeicher is being used in a general, non-specific sense here.

So:

• Wir nutzen Cloudspeicher = We use cloud storage

This is similar to English, where you often say we use cloud storage, not we use the cloud storage, unless you mean a specific one.

If you wanted to refer to a particular storage service or a specific system, you might use an article, for example:

• Wir nutzen den Cloudspeicher des Unternehmens.
  = We use the company’s cloud storage.

Yes. German very often forms nouns by combining smaller words into one compound noun.

Here:

• Cloud
  • Speicher = Cloudspeicher

This is extremely normal in German. English often writes similar ideas as two words, but German usually prefers one compound noun.

Other similar examples:

• Dateiformat = file format
• Speicherplatz = storage space
• Benutzerkonto = user account

So both Cloudspeicher and Dateiformat are good examples of typical German compound nouns.

It is the direct object of nutzen, so it is in the accusative case.

The verb nutzen usually takes an accusative object:

• Wir nutzen Cloudspeicher.
• Ich nutze das Programm.
• Sie nutzt den Dienst.

In your sentence, there is no article, so you do not see a special accusative ending on Cloudspeicher itself. But grammatically, it is still the accusative object.

Here passen means something like:

• to fit
• to be suitable
• to be compatible
• to work

So:

• das Dateiformat passt nicht

means that the file format is not suitable or does not fit the requirements.

Depending on context, English translations could be:

• the file format doesn’t fit
• the file format isn’t suitable
• the file format isn’t compatible

German uses passen quite broadly in situations where English might choose different verbs.

In a simple main clause, nicht usually comes later in the sentence, often after the verb if there is not much else following.

So:

• Das Dateiformat passt nicht.

is the natural word order.

A useful rough rule is:

• the conjugated verb stays in second position in a main clause
• nicht usually comes afterward unless it is specifically negating one element

Compare:

• Das Dateiformat passt nicht.
  = The file format doesn’t fit.
• Das Dateiformat passt nicht zum System.
  = The file format doesn’t fit the system.

So passt nicht is exactly what you would expect here.

Sometimes you can express a similar idea with wenn nicht, but it is not always the same in tone or structure.

• es sei denn is the standard fixed expression for unless
• wenn nicht is more literally if not

So for this sentence, es sei denn is the most natural choice.

For learners, the safest approach is:

• learn es sei denn as the normal translation of unless
• use wenn nicht only when you specifically mean if not

Yes. Das Dateiformat is a neuter noun.

So its article in the nominative singular is:

• das Dateiformat

In the clause:

• das Dateiformat passt nicht

das Dateiformat is the subject, so it is in the nominative case.

This also shows a useful contrast in the full sentence:

• Cloudspeicher = object of nutzen
• das Dateiformat = subject of passt

Yes, in many contexts both can work, but nutzen often sounds a bit more like make use of / use, while benutzen can sound slightly more concrete, like use in a practical sense.

So:

• Wir nutzen Cloudspeicher.
• Wir benutzen Cloudspeicher.

Both are understandable and natural, though nutzen is very common in technical, business, and formal contexts.

In this sentence, nutzen fits especially well because the context sounds general and somewhat technical.