Learn Hardcore Turkish: O, mesajı hemen yazıverdi ve gönderdi. - She quickly wrote the message and sent it.

It adds a nuance of quickness, ease, or spontaneity: "just wrote it," "wrote it in a snap," sometimes with mild surprise or casualness. It’s often called the quickness/effortlessness suffix.

How is yazıverdi built morphologically?

Breakdown: yaz + ıver + di

yaz = write (verb root)
-ıver- = quick/just (light-verb suffix from vermek)
-di = simple past, 3rd person singular Vowel harmony picks -ıver- after the back vowel a in yaz.

How would the meaning change if it were yazdı instead of yazıverdi?

yazdı is neutral "wrote (it)." yazıverdi implies it was done quickly/easily or offhand. Without -ıver-, you can still get the idea with adverbs: hemen yazdı, çabucak yazdı, bir çırpıda yazdı.

Can -iver be used with other verbs?

Yes. Examples:

açıverdi = opened it quickly
uyuyuverdi = nodded off all of a sudden
kopuverdi = it snapped (suddenly)
şöyle bir bakıver = just have a quick look Vowel harmony gives four forms: -ıver/-iver/-uver/-üver.

Why is mesajı marked with -ı? When would it be just mesaj?

Because it’s a definite direct object: "the message." Definite objects take accusative: mesajı. If it were indefinite, you’d leave it bare or use bir:

mesaj yazdı / bir mesaj yazdı = wrote a message.

Could mesajı mean "his/her message" here?

Not here. "His/her message" as a direct object would be mesajını (root mesaj + 3sg possessive -ı + buffer -n- + accusative -ı). Here mesajı is the accusative of "message." For possession: onun mesajını yazdı.

Do we need to repeat the object with onu before gönderdi?

No. Turkish normally omits a repeated definite object if it’s clear. ... yazıverdi ve gönderdi reads as "wrote it and sent it." You may say onu gönderdi for emphasis or clarity, but it’s not required.

Is O necessary, or can we drop it?

You can drop it: Mesajı hemen yazıverdi ve gönderdi. Subject pronouns are often omitted because the verb ending shows person/number. Keep O for emphasis, contrast, or to avoid ambiguity.

What about the comma after O — is it correct?

It’s optional and usually omitted. O mesajı... is more natural. A comma can mark a slight pause or topicalization, but many writers wouldn’t use it here.

What exactly does hemen contribute? Can it be placed elsewhere? Is it the same as hemen hemen?

hemen = right away, immediately.
Placement is flexible: O hemen mesajı yazıverdi..., O mesajı hemen yazıverdi..., Hemen mesajı yazıverdi... All fine; default is before the verb or just before the object.
hemen hemen = almost/nearly (e.g., hemen hemen herkes). It’s different.

Why is the past tense -di used? Could we use -miş instead?

-di = simple past with direct knowledge/certainty. yazıverdi, gönderdi states facts. -miş (yazıvermiş, göndermiş) marks hearsay/inference: "apparently/it seems he quickly wrote it and sent it."

Is using ve the best way to connect the two verbs? What about -ip?

Both are fine.

With ve: ... yazıverdi ve gönderdi keeps two full verbs, slightly more balanced/formal.
With -ip: ... yazıverip gönderdi is very common and implies sequence ("wrote it and then sent it") while subordinating the first action.

Can we also attach -iver to gönderdi?

Yes: ... mesajı hemen yazıverip gönderiverdi. That says both actions were done quickly/casually. With -iver only on the first verb, the quickness is focused on the writing.

How do we make negatives or questions with -iver?

Negative (usual): yazıvermedi = he didn’t just write it quickly.
Question: yazıverdi mi?, gönderiverdi mi?
Note: yazmayıverdi also exists, often meaning "he just didn’t (bother to) write it," a slightly different nuance from yazıvermedi.

Are there word-order variations for emphasis?

Yes:

Focus on the object: Mesajı hemen yazıverdi ve gönderdi.
Stronger immediacy: Hemen mesajı yazıverdi ve gönderdi.
Contrastive subject: Mesajı hemen o yazıverdi ve gönderdi. (It was he/she who did it.) Word order mainly changes emphasis, not basic meaning.

Is -iver formal or colloquial?

It’s very natural in speech and informal writing. In formal prose, writers often prefer adverbs like hemen, derhal, çabucak or simply omit the extra nuance.

How do I pronounce the undotted ı in yazıverdi and mesajı?

It’s a high back unrounded vowel with no direct English equivalent. Keep lips unrounded and the tongue back: yaz-ı-ver-di, me-sa-jı.