Learn Hardcore Turkish: O, herkesin farklı olabildiğini kabul ediyor. - She accepts that everyone can be different.

You can say the sentence with or without the pronoun:
- O, herkesin farklı olabildiğini kabul ediyor.
- Herkesin farklı olabildiğini kabul ediyor.

The second one (without O) is actually more natural, because the verb kabul ediyor already shows it’s third person singular: (o) kabul ediyor = “he/she accepts”.

About the comma:

The comma after O is optional here.
It’s sometimes used in writing for a tiny pause or emphasis: O, ... “As for him/her, …”
In most modern texts, you’d just write: O herkesin farklı olabildiğini kabul ediyor. or drop O completely.

Why is it herkesin and not just herkes?

Herkes means everyone / everybody.

Here we have herkesin because of the structure:

herkesin farklı olabildiği literally: the fact that everyone can be different / everyone’s being able to be different

In Turkish, when you make a noun clause with -dik/-diğ-, the subject of that clause usually takes genitive (-in) just like a possessor:

Ali’nin gelmesi = Ali’s coming / the fact that Ali comes
çocukların oynaması = the children’s playing / the fact that the children play
herkesin farklı olabildiği = everyone’s being able to be different / the fact that everyone can be different

So:

herkes → base form
herkesin → “of everyone / everyone’s” (genitive), used as the subject of the embedded clause.

How is farklı olabildiğini built up, and what does each part mean?

Morphological breakdown:

farklı = different
ol- = to be / to become
-a- = vowel of the abil- suffix (here as -a- for vowel harmony)
-bil- = can / be able to (potential suffix)
-diğ- (from -dik) = participle/nominalizer forming noun clauses
-i = its / his / her (3rd person singular possessive)
-n- = buffer consonant before a case ending
-i = accusative case

So:

farklı ol-a-bil-diğ-i-n-i
different be-can-DIK-3SG.POSS-ACC

Functionally, farklı olabildiğini =
“(the fact) that (he/she/everyone) can be different”.

The whole chunk herkesin farklı olabildiğini = “that everyone can be different” and acts as the object of kabul ediyor (“accepts / acknowledges”).

Why is it farklı olabildiğini and not farklı olabilir?

Farklı olabilir is a finite clause (“may be different / can be different”).
But in this sentence, we need a noun clause that can act as an object of kabul etmek.

Compare:

Herkes farklı olabilir.
“Everyone can be different.” (independent sentence)
O, herkesin farklı olabildiğini kabul ediyor.
“He/she accepts that everyone can be different.” (here “that everyone can be different” is a thing, an object)

To turn “can be different” into a noun-like phrase, Turkish uses the -dik/-diğ- structure:

farklı olabilir → farklı olabildiği = “the fact that (they) can be different”

Then we put it in accusative to mark it as the direct object:

farklı olabildiği → farklı olabildiğini = “(that) he/she/they can be different” (as an object).

What is the difference between farklı olduğunu and farklı olabildiğini?

Both are possible, but they’re not the same:

farklı olduğunu
- from farklı ol-duğ-u-nu
- meaning: that (they) are different
farklı olabildiğini
- from farklı ol-a-bil-diğ-i-n-i
- meaning: that (they) can be (or are able to be) different

Nuance:

farklı olduğunu kabul ediyor
→ “He/she accepts that everyone is different.” (a statement of fact)
farklı olabildiğini kabul ediyor
→ “He/she accepts that everyone can be different / is capable of being different.”
(focus on possibility / potential / capacity, not just a static fact)

What exactly does the -diğini part do in olabildiğini?

-dik / -diğ- is a very important suffix in Turkish. It turns a verb into something like an English “that…” clause or gerund.

Pattern:

gel-diğ-i = his/her coming / the fact that he/she comes
yap-tık-ları = the things they did / what they did
ol-a-bil-diğ-i = his/her being able to be / the fact that he/she can be

Then you can add case endings to this new noun-like form:

olabildiği = (its) being able to be / the fact that (it) can be
olabildiğini = accusative → “the fact that (it) can be” as a direct object
→ object of kabul ediyor

So -diğini here marks:

A noun clause derived from a verb (via -dik/-diğ-), and
That this clause is in accusative (via the final -i) and is being “accepted”.

What does the final -ni at the end of olabildiğini refer to?

The final -ni is really two pieces:

-n- = buffer consonant (used before case endings when there is already a possessive ending)
-i = accusative case

Structure:

olabildiği = “his/her/its being able to be” / “the fact that he/she/it can be”
- -i (before the buffer n) = 3rd person possessive (“its”)
olabildiğ-
- -i (3sg poss.) + -n (buffer) + -i (accusative)
  → olabildiğini

So:

The inner -i = “its / his / her” (ties to herkesin as the subject of the clause)
The outer -i (after -n-) = accusative, making the whole clause the object of kabul ediyor.

Why not say O, kabul ediyor ki herkes farklı olabilir instead?

You can say:

O, herkes farklı olabilir diye kabul ediyor.
O, herkes farklı olabilir ki demiyor (different structure)

But in standard, natural Turkish:

For verb + “that…” clause (like accept that…, know that…, think that…), the usual pattern is with -dik/-diğ- noun clauses:
- … bildiğini biliyor (knows that he knows)
- … geleceğini düşünüyor (thinks that he will come)
- … farklı olabildiğini kabul ediyor (accepts that (everyone) can be different)

Using ki to introduce “that”-clauses is possible but:

It often sounds more colloquial or has slightly different nuance/word order.
Here, herkesin farklı olabildiğini kabul ediyor is the most neutral and grammatically canonical way to say it.

So the -diğini structure is the “default” for such object clauses in standard written Turkish.

What is the difference between kabul ediyor, kabul eder, and kabul etti?

All three are from kabul etmek = “to accept / to acknowledge”.

kabul ediyor (present continuous / progressive)
- “is accepting / accepts (right now or in general with ongoing nuance)”
- In many contexts it’s just used as a normal present tense:
  - O, herkesin farklı olabildiğini kabul ediyor.
    = He/she accepts / acknowledges that everyone can be different.
kabul eder (aorist / habitual)
- Used for general facts, habits, typical behavior:
  - O, eleştiriyi kolay kolay kabul etmez.
    = He does not easily accept criticism.
- Could be:
  - O, herkesin farklı olabildiğini kabul eder.
    = As a rule / generally speaking, he/she accepts that everyone can be different.
kabul etti (simple past)
- Completed action in the past:
  - O, herkesin farklı olabildiğini kabul etti.
    = He/she accepted (finally acknowledged) that everyone can be different.

In your sentence, kabul ediyor suggests something like a current/ongoing or current-attitude type of acceptance.

Can I change the word order? For example, say O, kabul ediyor herkesin farklı olabildiğini?

Turkish word order is flexible, but not everything sounds equally natural.

Most natural here:

O, herkesin farklı olabildiğini kabul ediyor.
or without the pronoun:
Herkesin farklı olabildiğini kabul ediyor.

You can move the object to the front for focus:

Herkesin farklı olabildiğini, o kabul ediyor.
(“It is he/she who accepts that everyone can be different.” — emphasis on o.)

But:

O, kabul ediyor herkesin farklı olabildiğini.
– Grammatically understandable, but sounds odd and very untypical in standard speech.

Default rule:

The main verb usually comes at the end.
Long noun clauses like herkesin farklı olabildiğini normally appear before the main verb they belong to.

Why do we say herkesin farklı olabildiğini and not something like herkes farklı olabildiklerini?

Because of how herkes behaves grammatically:

herkes = “everyone, everybody”
Grammatically it is singular, even though it refers to many people.

So:

We treat herkes like a singular subject:
- herkes geliyor = everyone is coming (3rd singular)
- herkesin gelmesi = everyone’s coming / the fact that everyone comes
- herkesin farklı olabildiği = the fact that everyone can be different

We do not pluralize the verb noun here:

olabildikleri would suggest a plural subject like onlar (they), insanlar (people), etc.
But herkes itself stays singular.

Therefore:

herkesin farklı olabildiğini = correct
herkes farklı olabildiklerini = ungrammatical and confusing.

What tense or time does olabildiğini express? It looks like past (-di-) but means “can be”.

This is a common confusion.

The -diğ- in olabildiğini is from the -dik/-diğ- nominalizer, not from the simple past tense -di.

Compare:

oldu = (he/she) was / became (simple past)
olduğu = his/her being / that he/she is (nominalized clause)

So:

olabilir = can be / may be (finite verb)
olabildiği = (his/her) being able to be / that he/she can be
olabildiğini = that he/she can be (as object)

The tense/meaning (“can be” rather than “could be” etc.) is actually coming from:

The -abil- (ability/possibility) suffix, and
The context of the main verb kabul ediyor (present).

So:

-diğ- here does not mean past; it’s just forming a noun clause.
olabildiğini is best understood as “that (everyone) can be (different)” in terms of time/meaning.

Can we drop O altogether and just say Herkesin farklı olabildiğini kabul ediyor?

Yes, and that’s often more natural.

Turkish is a pro-drop language: subject pronouns are normally omitted when they are clear from context because the verb ending already shows the person/number.

(Ben) kabul ediyorum. = I accept.
(Sen) kabul ediyorsun. = You accept.
(O) kabul ediyor. = He/she accepts.

So:

O, herkesin farklı olabildiğini kabul ediyor.
Herkesin farklı olabildiğini kabul ediyor.

Both mean the same; the second is more typical. You only keep O when you want to emphasize the subject or there might be ambiguity about who you are talking about.