Takviye alerjiye neden olmadı.

Because the verb phrase neden olmak takes the thing affected in the dative case, which often corresponds to to in English.

• alerji = allergy
• alerjiye = to the allergy

So:

• Takviye alerjiye neden oldu = The supplement caused an allergy.
• Takviye alerjiye neden olmadı = The supplement did not cause an allergy.

The ending is -ye here because alerji ends in a vowel, so Turkish inserts a buffer y before the dative ending.

Literally, it is something like did not become a cause or was not a cause.

Breakdown:

• neden = cause, reason
• olmak = to be, to become
• olmadı = was not / did not become

Together, neden olmak is a common Turkish expression meaning to cause.

So even though the literal structure is different from English, the natural translation is simply:

• neden oldu = caused
• neden olmadı = did not cause

It comes from the verb olmak.

The steps are:

• ol- = verb stem
• -ma- / -me- = negative marker
• -dı / -di / -du / -dü = past tense

So:

• oldu = it was / it became
• olmadı = it was not / it did not become

Here the form is -ma- because of vowel harmony, and -dı gives the past tense.

Turkish usually does not use articles like English the and a/an.

So takviye can mean:

• supplement
• a supplement
• the supplement

and alerji can mean:

• allergy
• an allergy
• the allergy

The exact meaning depends on context.

If Turkish wants to emphasize a/an, it can use bir:

• Bir takviye alerjiye neden olmadı = A supplement did not cause an allergy.

But in many ordinary sentences, bir is not necessary.

Takviye has a few possible meanings depending on context, such as:

• supplement
• reinforcement
• support
• boost

In a health or nutrition context, it usually means supplement, as in a dietary supplement.

So in this sentence, takviye most naturally means the supplement or a supplement.

Because Turkish expresses this idea with the fixed expression bir şeye neden olmak, literally to be a cause to something.

That means the noun after it is not a direct object in the accusative case. Instead, it takes the dative:

• alerjiye = to an allergy

This is just how the Turkish construction works. Many Turkish verbs and verb phrases require a particular case, and learners often just have to memorize the pattern.

A useful pattern is:

• X, Y’ye neden oldu = X caused Y
• X, Y’ye neden olmadı = X did not cause Y

Yes. Turkish word order is flexible, although the most neutral order is often:

Subject + other elements + verb

So:

• Takviye alerjiye neden olmadı.

is a normal, neutral sentence.

But Turkish can move words around for emphasis. For example:

• Alerjiye takviye neden olmadı.
• Neden olmadı takviye alerjiye. — grammatically possible in special contexts, but not neutral and not something a learner should imitate early on.

In ordinary usage, keeping the verb at the end is the safest choice.

Not in standard Turkish. The usual expression is:

• sebep olmak = to cause

So you would say:

• Takviye alerjiye sebep olmadı is not correct in standard usage.
• Takviye alerjiye sebep olmadı sounds incomplete because sebep normally needs olmak with it.

The correct version is:

• Takviye alerjiye sebep olmadı — actually, this still lacks olmak as a separate lexical base? In Turkish the past form is built directly on olmak, so the correct full expression is:
• Takviye alerjiye sebep olmadı.

To clarify the pattern:

• sebep olmak and neden olmak both mean to cause
• Their past negative forms are:
  • sebep olmadı
  • neden olmadı

So yes, you can say:

• Takviye alerjiye sebep olmadı.

and it means almost the same thing as the original sentence.

They are very close in meaning, and both can mean to cause.

A rough guide:

• neden olmak is very common and neutral
• sebep olmak is also common and can sound slightly more formal in some contexts

In many sentences, they are interchangeable:

• Takviye alerjiye neden olmadı.
• Takviye alerjiye sebep olmadı.

Both are natural.

Because Turkish usually does not need subject pronouns when the subject is already clear.

Here, the subject is explicitly given as takviye, so there is no reason to add a pronoun.

Even when the subject is only understood from the verb, Turkish often leaves the pronoun out:

• Gelmedi. = He/she/it did not come.

In your sentence, takviye already tells us what did not cause the allergy.

Yes. Olmadı is the third person singular past negative form.

Since takviye is singular, the verb is singular too:

• Takviye ... olmadı. = The supplement did not ...

If the subject were plural, Turkish often still uses the singular verb in many contexts, though a plural verb is also possible depending on style and emphasis.

For example:

• Takviyeler alerjiye neden olmadı. = The supplements did not cause an allergy.

This is completely normal.

The y is a buffer consonant.

Turkish does not usually like two vowels directly next to each other when a suffix is added. Since alerji ends in a vowel and the dative ending also begins with a vowel, Turkish inserts y:

• alerji + e → alerjiye

You will see this in many words:

• masa + a → masaya
• anne + e → anneye
• su + a → suya

So the y is not part of the root; it is just there to make pronunciation smoother.