Learn Hardcore Turkish: Ben mutfakta soğanı tavada kavuruyorum.

Because it’s a definite direct object. Turkish marks specific/definite objects with the accusative suffix -I (which appears as -ı/-i/-u/-ü by vowel harmony). So:

soğanı = the onion (a specific onion)
soğan (no ending) = onion, some onion (non-specific/indefinite)

What if I don’t mean a specific onion?

Leave the accusative off:

Soğan kavuruyorum. = I’m sautéing onion / some onion. You can add quantity words:
Biraz soğan kavuruyorum. = I’m sautéing a bit of onion.
İki soğan kavuruyorum. = I’m sautéing two onions.

Why do both mutfakta and tavada use the same ending? Can a sentence have two locatives?

Yes. The locative suffix -DA (da/de/ta/te) expresses “in/at/on.” Here, it separately locates two different things:

Ben mutfakta = I am in the kitchen.
Soğan tavada = The onion is in the pan. Putting them together is natural: you’re in the kitchen; the onion is in the pan.

How do I choose between -da/-de and -ta/-te?

Two rules:

Vowel harmony (a/e): last vowel back (a, ı, o, u) → -da/-ta; front (e, i, ö, ü) → -de/-te.
Voicing (d/t): after a voiceless consonant (p, ç, t, k, f, s, ş, h), use t; otherwise d. Examples:
mutfak → mutfak + DA → mutfakta (k is voiceless → t; a is back → a)
tava → tava + DA → tavada (v is voiced → d; a is back → a)
ev → evde
ofis → ofiste

Do I need to keep the pronoun Ben?

No. The verb ending already shows the subject. Kavuruyorum means “I am sautéing.” Keeping Ben adds emphasis or contrast: Ben (I, as opposed to someone else) am doing it.

Why is it Ben, not Beni?

Ben is the subject (nominative). Beni is the accusative (“me”) and is used only for objects: e.g., Beni gördün = You saw me.

Is the word order fixed? Can I say Tavada soğanı kavuruyorum or Mutfakta soğanı tavada kavuruyorum?

Turkish word order is flexible, but the default is S-(time)-(place)-O-V. Your sentence is natural. You can also say:

Tavada soğanı kavuruyorum. (slight emphasis on “in the pan”)
Mutfakta soğanı tavada kavuruyorum. (fronts “in the kitchen”) Elements placed right before the verb often carry focus.

How is kavuruyorum formed?

Verb root: kavur- (to sauté/roast)
Present continuous: -yor (written with a harmony vowel: -ıyor/-iyor/-uyor/-üyor)
Personal ending: -um (1st person singular) Last vowel of the stem is u, so choose -uyor: kavur-uyor-um → kavuruyorum.

Harmony guide for -yor:

a/ı → -ıyor (yaz-ıyor-um → yazıyorum)
e/i → -iyor (gel-iyor-um → geliyorum)
o/u → -uyor (okur-uyor-um → okuruyorum → okunuyor? Note: with “oku-”: okuyor-um → okuyorum)
ö/ü → -üyor (gör-üyor-um → görüyorum)

How do I pronounce the odd letters in soğanı?

ğ (yumuşak g) doesn’t make a hard g sound; it lengthens/smooths the preceding vowel. In soğanı, the o/a transition is lengthened: roughly “so-ā-nı.”
ı is a dotless i, like a relaxed “uh” sound. soğanı ends with that sound: “so-ā-nuh.”

Is kavurmak the same as kızartmak or sotelemek?

Not exactly:

kavurmak: to brown/sauté/roast in a pan, often with little oil, until fragrant/browned (e.g., onions).
sotelemek: to sauté quickly over high heat (loan from French “sauter”).
kızartmak: to fry (pan-fry or deep-fry) in oil.
pişirmek: to cook (general).

How do I make it negative or ask a yes–no question?

Negative: insert -mA- before -yor; with harmony it becomes -mıyor/-miyor/-muyor/-müyor.
- Soğanı tavada kavurmuyorum. = I’m not sautéing the onion in the pan.
Yes–no question: add the particle mi (harmonized) separately.
- Soğanı tavada kavuruyor muyum? = Am I sautéing the onion in the pan?

How do plurals work for objects?

Specific plural object: add plural and accusative: soğanları = the onions (specific set).
- Soğanları tavada kavuruyorum.
Indefinite quantity: usually no plural, no accusative: soğan.
- Soğan kavuruyorum.
With numbers: plural is not used on the noun: iki soğan, not “iki soğanlar.”

Why is it mutfakta, not “mutfakda”?

Because the locative uses -ta after a voiceless consonant like k. Also note: with a vowel-starting suffix, final k often softens to ğ (e.g., dative: mutfağa), but with consonant-starting suffixes like -ta, it stays k: mutfakta.

Can I replace soğanı with a pronoun?

Yes, for a known/definite object use onu (“it” in accusative):

Onu tavada kavuruyorum. = I’m sautéing it in the pan.

What if I want to say “into the pan,” not “in the pan”?

Use the dative -A for motion toward: tavaya = into/onto the pan.

Putting the onion into the pan: Soğanı tavaya koyuyorum.
Locating the action in the pan (your original sentence) uses the locative tavada.

Could the present continuous here also express a scheduled near-future?

Yes. Turkish -yor can describe planned future arrangements:

Yarın mutfakta soğanı tavada kavuruyorum. = I’m sautéing the onion in the pan tomorrow. (arranged plan) For a neutral future, use -acak/-ecek: kavuracağım.