Learn Hardcore Turkish: Komşumu yarın misafir etmek istiyorum.

The final -u is the accusative case marker, used for a specific/definite direct object. Since you’re talking about a particular person (your neighbor), Turkish marks it with accusative: komşu + (my) -m + (ACC) -u → komşumu.

How exactly is Komşumu built? What’s inside that word?

komşu = neighbor (stem)
-m = 1st person singular possessive (my)
-u = accusative case (specific direct object) Because the possessive suffix ends in a consonant (m), you add accusative directly as -u (no buffer letter). Vowel harmony picks u (last vowel of komşu is u, so back/rounded).

Why isn’t it komşuyu here?

komşuyu = “the neighbor” (definite, but not possessed)
komşumu = “my neighbor” (definite and possessed) Use komşuyu if you mean a specific neighbor previously known from context, but not necessarily yours. Use komşumu when you mean your own neighbor.

When do we use the buffer letter y with the accusative?

You use buffer y before accusative -(I) if the word ends in a vowel:

komşu
- -(y)u → komşuyu But after the possessive -m (a consonant), you don’t need it:
komşu + m + u → komşumu

What does the expression misafir etmek mean as a verb?

It’s a light-verb construction meaning “to host,” literally “to make [someone] a guest.” Natural synonyms:

ağırlamak (very common, “to host/entertain”)
konuk etmek (using the synonym konuk for “guest”) All take a direct object in the accusative when specific.

Why is it etmek and not yapmak with misafir?

Many nouns (especially of Arabic origin) form verbs with etmek in Turkish (e.g., teklif etmek, yardım etmek). Misafir yapmak isn’t idiomatic for “to host”; the set phrase is misafir etmek.

Is the word order with yarın okay? Could I move it?

Yes. Common options:

Neutral/very common: Yarın komşumu misafir etmek istiyorum.
Given sentence (also fine): Komşumu yarın misafir etmek istiyorum. (slight emphasis on the object)
Strong focus on time by placing it near the verb end: Komşumu misafir etmek istiyorum yarın. (less neutral) Turkish is flexible, but moving elements changes focus/emphasis.

Could I say Komşumu yarın misafir edeceğim instead?

Yes, but it changes the meaning from desire to plan/promise:

… etmek istiyorum = I want/intend to do it.
… edeceğim = I will do it (plan/commitment).

Why istiyorum and not isterim?

istiyorum (present continuous form) = a current, concrete desire/intention.
isterim (aorist) = general tendency/preferences or polite/softer tone (“I would like”). Here, istiyorum fits a present desire about a specific plan for tomorrow.

Do I need to say … etmeyi istiyorum instead of … etmek istiyorum?

Both are correct:

… etmek istiyorum is very common and perfectly natural.
… etmeyi istiyorum (verbal noun + accusative) puts a bit more emphasis on the action as a thing you want. In everyday speech, -mek istemek is more frequent.

How would I make it negative or a yes/no question?

Negative: Komşumu yarın misafir etmek istemiyorum.
Yes/no question (about wanting): Komşumu yarın misafir etmek istiyor musun? Note the question particle mi/mı/mu/mü is written separately and agrees by vowel harmony (musun here).

What if I mean “my neighbors” (plural)?

Use plural + possessive + accusative: komşularımı.

Komşularımı yarın misafir etmek istiyorum. = I want to host my neighbors tomorrow.

How do I say “a neighbor” instead of “my neighbor”?

Non-specific: Yarın bir komşu misafir etmek istiyorum. (“a neighbor,” not a particular one)
Specific/identifiable (rarer with “bir”): Yarın bir komşuyu misafir etmek istiyorum. With possessed nouns, Bir komşumu… often means “one of my neighbors.”

How do I pronounce yarın and ş in komşu?

yarın uses the dotless ı (IPA roughly [ɯ]): not like English “i,” more like a close, back, unrounded vowel.
ş is “sh,” so komşu sounds like “kom-shu.”

Do I need to say Ben at the start?

No. The verb ending -um in istiyorum already shows the subject is “I.” Adding Ben adds emphasis or contrast:

Ben komşumu yarın misafir etmek istiyorum (as opposed to someone else).