Das Fotoalbum enthält alte Fotoabzüge.

Das is the definite article the for a neuter singular noun.

Here, Fotoalbum is neuter: das Fotoalbum.
Because Fotoalbum is the subject of the sentence, it appears in the nominative case:

• das Fotoalbum = the photo album

Also, Das is capitalized here simply because it is the first word of the sentence. In the middle of a sentence, it would normally be das.

German very often forms compound nouns by joining words together.

So:

• Foto = photo
• Album = album
• Fotoalbum = photo album

In English, we usually write this as two words, but in German it is normal to write it as one word.

The same thing happens with Fotoabzüge:

• Foto = photo
• Abzug = print / photographic print
• Fotoabzug = photo print

Because enthält is the conjugated form, while enthalten is the infinitive.

The infinitive is:

• enthalten = to contain

But in a normal present-tense main clause, the verb must be conjugated to match the subject.
The subject here is das Fotoalbum, which is third person singular, so the verb becomes:

• das Fotoalbum enthält = the photo album contains

This is like English to contain becoming contains.

This verb is irregular in the present tense. It follows the pattern of halten.

Compare:

• ich halte = I hold
• er hält = he holds

And similarly:

• ich enthalte = I contain / include
• es enthält = it contains

So the vowel changes from a to ä in the third person singular (and also in du forms).

It is accusative plural.

Why? Because enthält takes a direct object: the thing that is contained.
In this sentence:

• Das Fotoalbum = subject
• enthält = verb
• alte Fotoabzüge = direct object

A direct object in German is usually in the accusative case.

Since Fotoabzüge is plural, this phrase is accusative plural.

Because the adjective ending is determined by the case, gender, number, and article structure.

Here, there is no article before the noun phrase:

• alte Fotoabzüge

When there is no article, German usually uses strong adjective endings.
For plural accusative with no article, the adjective ending is -e:

• alte Fotoabzüge

If there were a definite article, you would get:

• die alten Fotoabzüge

So both are correct, but the endings are different because the structure is different.

German often leaves out an article in plural when the meaning is general or indefinite.

So:

• alte Fotoabzüge = old photo prints / some old photo prints

This is similar to English, where we can say:

• It contains old photo prints

without needing the or some.

If you wanted to make it definite, you could say:

• Das Fotoalbum enthält die alten Fotoabzüge.

That would mean the old photo prints, referring to specific ones.

The singular is:

• der Fotoabzug = the photo print

The plural is:

• die Fotoabzüge = the photo prints

You can tell it is plural here because:

1. the noun ends in -e
2. the vowel changes from u to ü
3. the context fits a plural object

So:

• Fotoabzug → Fotoabzüge

This is a common German plural pattern: umlaut + -e.

Not exactly.

• Foto usually means photo / photograph in a general sense.
• Fotoabzug is more specific: it means a photographic print, especially a physical printed copy.

So Fotoabzüge suggests actual printed photos, not just images in general.

Because they are nouns, and in German all nouns are capitalized.

So in this sentence:

• Fotoalbum is a noun
• Fotoabzüge is a noun

That is why they begin with capital letters.

This is one of the most noticeable differences between German and English spelling.

Yes. This is a very normal main-clause word order:

• Das Fotoalbum = subject
• enthält = verb
• alte Fotoabzüge = object

So the pattern is:

• Subject – Verb – Object

In German main clauses, the conjugated verb normally appears in second position.
This sentence is a straightforward example of that rule.