Мы выберем песню, если только каталог песен её не включает.

Not here.

In this sentence, если только means something like unless or except if.

So this is not the emotional English if only used for wishes, as in If only I knew. Russian если только can sometimes look similar word-for-word, but in this kind of sentence it has the function of setting an exception.

A useful pattern to remember is:

если только ... не = unless

Because Russian often expresses unless with the pattern если только ... не.

So the не is not just a random extra negative. It is part of how this construction works. English uses a single word, unless, while Russian often uses a longer structure with если только plus a negated verb.

So learners should understand the whole chunk:

если только каталог песен её не включает
= unless the song catalog includes it

Do not translate it mechanically as if only ... not.

Выберем is the 1st person plural future form of the perfective verb выбрать.

That tells you two things:

• мы = we
• выберем = we will choose

The perfective aspect is used because the action is seen as a single completed result: making a choice.

Compare:

• мы выберем = we will choose / we will make the choice
• мы будем выбирать = we will be choosing / we will spend time choosing
• мы выбираем = we choose / we are choosing

So выберем is the natural form if the sentence is about one completed future choice.

Because включает describes a state or fact about the catalog: whether it includes the song.

Russian often uses the present tense for this kind of general or current fact, even when the main clause is about a future decision.

So the logic is:

• we will choose
• depending on whether the catalog includes it

The inclusion is treated as a fact being checked, not necessarily as a future action of adding something.

If you wanted a different meaning, such as a future act of including something, you might use another form, but that would change the nuance.

Because песню is the direct object of выберем.

The verb выбрать takes the accusative case, so песня becomes песню.

This is a regular feminine singular pattern:

• nominative: песня
• accusative: песню

So:

• песня = a song as the subject
• выбрать песню = to choose a song

Её refers back to песню.

In English, we would say it, because song is inanimate. In Russian, pronouns agree with the grammatical gender of the noun they refer to, so a feminine noun like песня is referred to with её.

Also, её is in the accusative here because it is the direct object of включает.

Compare:

• она = she / it as a subject
• её = her / it as an object

So you cannot use она here, because она is nominative, not an object form.

Also note that её is often written as ее in normal typing. They are the same word.

Because Russian commonly uses the genitive after a noun like каталог to mean catalog of ....

So:

• каталог песен = catalog of songs

Here песен is the genitive plural of песня.

This is a very common Russian pattern:

• список книг = list of books
• группа студентов = group of students
• каталог песен = catalog of songs

So песен is the normal form after каталог.

Yes, the word order can vary.

Russian word order is more flexible than English word order, because case endings show grammatical relationships. In this sentence, каталог песен её не включает is understandable and grammatical.

However, many speakers may find каталог песен не включает её a little more neutral in ordinary prose.

The difference is mostly about emphasis:

• её не включает can put a bit more focus on it
• не включает её can sound more neutral

So the version you have is possible, but word order in Russian often carries nuance rather than changing the basic meaning.

It is grammatical, but it may sound a little formal or slightly heavy, especially because of the combination of если только and включает.

A native speaker might choose a clearer or lighter rephrasing depending on context, for example with a structure meaning only if it is not in the catalog or unless it is included in the catalog.

Still, as a learning example, the sentence is useful because it shows several important points at once:

• perfective future: выберем
• accusative object: песню
• genitive after a noun: каталог песен
• object pronoun: её
• the если только ... не pattern for unless

So even if a native speaker might phrase it differently in everyday speech, the grammar is legitimate.

In this exact structure, usually no.

The verb включать normally needs an object, so каталог песен не включает by itself sounds incomplete unless the object is extremely obvious from the broader context.

That is why её is useful here: it clearly points back to песню.

Russian does sometimes omit things that are understood from context, but with a transitive verb like включать, keeping the pronoun is the safer and more natural choice unless you rewrite the sentence in a different way.