Learn Hardcore Turkish: Siz müsaitseniz, belgeleri yarın size göstertmek istiyorum. - If you are available, I would like to have you shown the documents tomorrow.

Göstermek = to show (you do the showing).
Göstertmek = to have someone show / to get shown (you arrange for someone else to show).
So Belgeleri yarın size göstertmek istiyorum means you plan to have a third person show the documents to the listener. If you yourself will show them, say Belgeleri yarın size göstermek istiyorum.

So who actually does the showing here?

Not the speaker. The speaker is the causer; the actual shower (the causee) is unspecified. The recipient is size (to you). If you want to name the causee, put them in the dative: Asistanıma belgeleri yarın size göstertmek istiyorum = I want to have my assistant show you the documents tomorrow.

Why is belgeleri in the accusative? Could it be without -i?

The -i (accusative) marks a definite direct object: belgeleri = the documents. If the object is indefinite, you drop the accusative: yarın size belge göstertmek istiyorum or bazı belgeler göstertmek istiyorum.

Why is it size and not sizi?

With göstermek/göstertmek, the thing shown is the direct object (accusative) and the person it is shown to takes the dative: X’i Y’e göstermek. So it’s belgeleri (accusative) and size (dative ‘to you’). Sizi would mean “you” as the direct object (e.g., Sizi müdüre göstermek = to show you to the manager).

Can I drop the initial Siz? And what about sen vs siz here?

Yes, you can drop it: Müsaitseniz, … The person is already encoded in -seniz.
Politeness: siz is the polite/plural “you,” sen is informal singular. With sen, you’d say Müsaitsen, belgeleri yarın sana göstertmek istiyorum.

How is müsaitseniz formed? Could I say müsait iseniz?

Yes. Formation: müsait + conditional -se/-sa + 2nd person plural/polite -niz → müsaitseniz. The fuller form müsait iseniz is also correct (a bit more formal/written). You may add Eğer for emphasis: Eğer müsaitseniz, …

Is müsaitseniz or müsait olursanız better? What’s the nuance?

Both work. Müsaitseniz = if you are available (at the relevant time). Müsait olursanız = if you (happen to) become available / if you will be available. With a time adverb like “tomorrow,” many speakers prefer Yarın müsaitseniz, …

Where should I put yarın?

Time adverbs typically go early: Yarın size belgeleri göstertmek istiyorum. If you mean “if you are free tomorrow,” attach it to the if-clause: Yarın müsaitseniz, belgeleri size göstertmek istiyorum. Your original word order is fine, just a bit less tight semantically.

Is istiyorum the right choice? Would isterim or istiyordum/isterdim be more polite?

İstiyorum states a concrete current intention. İsterim is less forceful and can sound more like a polite offer. İstiyordum/İsterdim softens it further (common in requests). For example: Yarın müsaitseniz, belgeleri size göstertmek isterim/istiyordum/isterdim. You can also soften with mümkünse, rica etsem, etc.

Is the comma and the word eğer required in the if-clause?

Use a comma when the conditional clause comes first. Eğer is optional and adds emphasis: (Eğer) müsaitseniz, …

Can I avoid the causative by using a passive?

Yes: Belgelerin yarın size gösterilmesini istiyorum. Literally, “I want the showing of the documents to you tomorrow.” It’s neutral and avoids mentioning who will do the showing.

How do I explicitly say who will do the showing?

Use the causative and mark the causee in the dative: Asistanıma belgeleri yarın size göstertmek istiyorum. Here asistanıma (to my assistant) is the causee; size is the recipient; belgeleri is the direct object.

Does göstertmek sound too heavy or rare?

No. It’s standard when you mean “arrange for someone else to show.” Examples: Patron müşteriye raporu göstertti. / Müdür, sekreterine dosyaları yeni çalışanlara göstertti. It only feels heavy if you chain multiple causatives.

Could I use göstereyim instead?

Göstereyim (optative) means “let me show (myself).” So Müsaitseniz, belgeleri yarın size göstereyim is a polite offer to personally show the documents. It changes the meaning from arranging someone else to do it.

Why both Siz and size—is that redundant?

They play different roles. Initial Siz is the subject of the if-clause (Siz müsaitseniz = if you are available), while size is the indirect object in the main clause (to you). Not redundant.

Any quick synonyms for müsaitseniz that fit here?

Yes: uygunsanız, vaktiniz varsa, zamanınız olursa. For example: Uygunsanız, belgeleri yarın size göstertmek istiyorum.