Learn Hardcore Turkish: Onu ikna etmek için şüphelerini tek tek yanıtladım. - I answered her doubts one by one to persuade her.

Purpose clause: Onu ikna etmek için = “in order to persuade him/her.” The verb here is the infinitive compound ikna etmek.
Main clause: şüphelerini tek tek yanıtladım = “I answered (his/her) doubts one by one.” The finite verb is yanıtladım.

The object onu belongs to the purpose clause (it’s the object of ikna etmek), not to the main clause.

Who is the subject? Why is ben not written?

The subject is “I,” because the verb yanıtladım ends with -dım (past tense + 1st person singular). Turkish usually drops subject pronouns; ben is only added for emphasis or contrast.

Why is it onu (accusative) and not ona (dative)?

Because ikna etmek (“to persuade/convince”) takes a direct object in the accusative. So you persuade someone = birini ikna etmek. The dative (ona) would be wrong here.

Can I say onu ikna olmak için?

No. ikna etmek = “to convince (someone),” while ikna olmak = “to be convinced (oneself).” You want the causative meaning, so use ikna etmek.

What does için do here, and why is the verb in the infinitive? Can I use other forms?

için means “for / in order to” and it takes a noun phrase. To turn a verb into a noun phrase, Turkish uses the infinitive -mAk. Here, the compound ikna etmek appears in its infinitive form.
Alternatives:
- … etmek üzere (a bit more formal/literary): Onu ikna etmek üzere…
- … diye (colloquial-purpose): Onu ikna edeyim diye… (note the subjunctive-like form with intention)
- … amacıyla/maksadıyla (formal): Onu ikna etmek amacıyla…

What exactly does şüphelerini mean, and why that ending?

It’s “his/her doubts” as a definite direct object. Morphology:

şüphe = doubt
-ler = plural → şüpheler
-i = 3rd person possessive (“his/her”) → şüpheleri
-(n)I = accusative case for definite objects; because there’s already a 3rd person possessive, a buffer n is used → şüphelerini

So şüphelerini = “(his/her) doubts” (accusative, definite).

Is şüphelerini ambiguous? Could it also mean “your doubts”?

Yes, in isolation şüphelerini can also mean “your (sg) doubts” (2nd person possessive + accusative). Context usually disambiguates. If needed, make it explicit:

onun şüphelerini = his/her doubts
senin şüphelerini = your (sg) doubts
sizin şüphelerinizi = your (pl/polite) doubts

Why do we need the accusative on şüphelerini? What changes if I drop it?

Turkish marks definite/specific direct objects with accusative. Here, we’re talking about that person’s actual, specific doubts, so -(n)I is required.

With accusative: şüphelerini yanıtladım = “I answered (those) doubts.”
Without accusative: şüpheleri yanıtladım tends to sound generic/non-specific or habitual (“I answer his/her doubts (as a general activity)”), and is pragmatically odd for a one-off past action.

Do I need to add onun before şüphelerini?

Not if the context already makes it clear whose doubts they are (and here, onu in the purpose clause points to the same person). Add onun only if there’s any risk of ambiguity.

Why use yanıtlamak? How does it compare to cevaplamak and cevap vermek?

yanıtlamak = “to answer” (transitive, direct object): Şüphelerini yanıtladım.
cevaplamak = also “to answer” (transitive): Şüphelerini cevapladım.
cevap vermek = “to give an answer” (light-verb phrase) and takes the dative: Şüphelerine cevap verdim. All are natural; yanıtlamak sounds a bit more native/modern-register; cevap words are of Arabic origin but very common.

Could I say yanıt verdim instead of yanıtladım?

Yes, but switch the case:

Şüphelerini yanıtladım. (accusative with yanıtlamak)
Şüphelerine yanıt verdim. (dative with yanıt vermek)

What does tek tek mean exactly? Any synonyms? Is it the same as teke tek?

tek tek = “one by one, individually.”
Synonyms: teker teker, birer birer.
Don’t confuse with teke tek, which means “one-on-one” (as in a duel or a one-on-one meeting), not “one by one.”

Where can I place tek tek in the sentence?

Adverbs are flexible. Common options:

Şüphelerini tek tek yanıtladım. (as given)
Tek tek şüphelerini yanıtladım. Both are natural; initial position adds slight emphasis to the manner (“one by one”).

Is a comma needed after Onu ikna etmek için?

Optional. With a short purpose phrase, many writers omit the comma. With longer or heavier initial phrases, a comma helps readability: Onu ikna etmek için, şüphelerini tek tek yanıtladım.

Aren’t there two accusatives here (onu, şüphelerini)? Is that allowed?

They belong to different clauses, so it’s fine:

onu is the object of the subordinate verb ikna etmek.
şüphelerini is the object of the main verb yanıtladım. Having two accusatives within a single simple clause is generally not allowed, but across clauses it’s normal.

Could I say onu ikna edebilmek için? What nuance does that add?

Yes. … edebilmek için adds an “ability”/“manage to” nuance: “so that I could manage to persuade him/her.” It hints that persuading might be difficult.

Are there more idiomatic collocations than şüphelerini yanıtladım?

You can certainly say it, but many speakers might also use:

Şüphelerini gidermek için… = “to dispel/alleviate (his/her) doubts…”
Kaygılarını/tereddütlerini gidermek… if the nuance is “anxieties/hesitations.”

Any tips on pronouncing ikna, şüphe, and yanıtladım?

ikna: two syllables, “ik-na” (no extra vowel between k and n).
şüphe: “şüp-he” (the h is pronounced; ü is like German/French ü).
yanıtladım: “ya-nıt-la-dım” (stress typically near the end in Turkish; the ı is the undotted ı sound).