İçten teşekkür ederim.

In this sentence, it works adverbially.

Its basic meaning is something like sincere / heartfelt / from the heart, but here it modifies the act of thanking, so the sense is I thank you sincerely rather than the thanks are sincere.

This is common in Turkish: a word that looks adjective-like in English can often be used directly in an adverb-like way, without needing a separate -ly form.

Because Turkish has two different I letters:

• İ / i = a dotted letter
• I / ı = a dotless letter

These are separate letters in the alphabet, not just spelling variants.

So when içten is capitalized, it becomes İçten, not Içten.

Turkish usually expresses this idea with the fixed expression teşekkür etmek.

So:

• teşekkür = thanks / gratitude
• etmek = to do, to make, to perform

Together, teşekkür etmek means to thank / to express thanks.

This kind of structure is very common in Turkish. Other examples are:

• yardım etmek = to help
• telefon etmek = to call on the phone
• dua etmek = to pray

So the whole unit teşekkür ederim functions like the English verb I thank.

Ederim is the 1st person singular form of etmek here, meaning I do / I make, as part of teşekkür etmek.

In this expression, teşekkür ederim is the normal polite way to say thank you.

Grammatically, it is the aorist form, but in Turkish the aorist is often used in polite formulas that describe what the speaker is doing right now. So even though English learners may expect something like I am thanking, Turkish naturally uses teşekkür ederim.

That is why:

• teşekkür ederim = the standard polite formula
• teşekkür ediyorum is possible, but it sounds less like the fixed everyday formula

Because Turkish usually drops subject pronouns when the verb already shows the person.

In ederim, the ending -im already tells you the subject is I.

So:

• teşekkür ederim = I thank / thank you
• ben teşekkür ederim = I thank you, with extra emphasis

You only add ben when you want contrast, emphasis, or clarification.

It can be omitted if it is already clear from the situation.

If you want to include it, Turkish uses the dative case:

• Sana içten teşekkür ederim. = to you singular/informal
• Size içten teşekkür ederim. = to you plural or polite singular

So the sentence without sana/size is completely natural when the listener is obvious.

It is polite and a little more heartfelt or elevated than plain teşekkür ederim.

You might hear or use it in:

• speeches
• emails
• letters
• formal thanks
• emotional or sincere situations

In casual conversation, people more often say:

• teşekkür ederim
• teşekkürler
• çok teşekkürler
• sağ ol / sağ olun depending on the situation

So İçten teşekkür ederim is natural, but it sounds a bit more deliberate and sincere than everyday quick thanks.

Turkish word order is flexible, but some versions sound more natural than others.

These are natural:

• İçten teşekkür ederim.
• Size içten teşekkür ederim.
• Size teşekkür ederim.

But something like Teşekkür ederim içten sounds unusual in normal speech.

So for a learner, the safest pattern is to keep içten before teşekkür ederim.

A rough guide is:

• İçten ≈ ich-TEN
• teşekkür ≈ te-shek-KYÜR
• ederim ≈ eh-de-REEM

A few useful sound notes:

• ç = ch as in church
• ş = sh as in ship
• r in Turkish is usually lighter than in English
• ü has no exact English equivalent; it is pronounced with rounded lips, somewhat like German ü

So the sentence sounds roughly like:

ich-TEN te-shek-KYÜR eh-de-REEM

They emphasize different things.

• Çok teşekkür ederim emphasizes amount/intensity: many thanks, thanks a lot
• İçten teşekkür ederim emphasizes sincerity: heartfelt thanks, sincere thanks

So:

• çok = stronger in quantity
• içten = stronger in emotional sincerity

Both are polite, but they are not exactly the same in tone.