Anahtarı içeride unutunca arka kapıyı açamadım.

Anahtar means key. In this sentence, anahtarı is the direct object of unutmak (to forget / leave behind).

The ending -ı is the accusative case marker, which is often used for a specific or definite direct object. So:

• anahtar = a key / key
• anahtarı = the key / a specific key as the object

So anahtarı içeride unutunca is literally something like when/since [I] forgot the key inside.

This is a very common question.

• içeri usually means inside / inward / into the inside and suggests direction or movement.
• içeride means inside in the sense of location.

Here the key is located inside, not moving inward, so içeride is the natural choice.

Compare:

• Anahtarı içeri koydum. = I put the key inside.
  movement toward the inside
• Anahtar içeride. = The key is inside.
  location

Unutunca comes from:

• unut- = verb root, forget
• -unca / -ince = a suffix meaning when, once, after, and sometimes since/because

So unutunca means something like:

• when [I] forgot
• once [I] forgot
• since/because [I] forgot

In this sentence, it has a causal sense:
Because I left/forgot the key inside, I couldn’t open the back door.

Yes. In Turkish, unutmak is often used where English would naturally say leave by mistake.

So:

• Anahtarı içeride unuttum.

can be understood as:

• I forgot the key inside.
• I left the key inside.

In English, leave is often more natural here, but Turkish commonly uses unutmak.

Açamadım breaks down like this:

• aç- = open
• -a- = ability/potential element
• -ma- = negation
• -dı- = past tense
• -m = I

So açamadım means:

• I could not open
• I wasn’t able to open

This is important because it is not the same as açmadım.

• açmadım = I didn’t open
• açamadım = I couldn’t open

The second one expresses inability, not just the fact that the action didn’t happen.

For the same basic reason as anahtarı: it is a specific direct object.

• kapı = door
• kapıyı = the door as a direct object

The y is a buffer consonant. Turkish often inserts y before a vowel-initial suffix when the noun already ends in a vowel.

So:

• kapı + -ı would be awkward
• Turkish makes it kapıyı

And arka kapıyı means the back door as the thing that could not be opened.

Because Turkish often leaves out subject pronouns when the verb already shows the person clearly.

In açamadım, the ending -m tells you the subject is I.

So Turkish does not need ben unless the speaker wants emphasis or contrast.

• Açamadım. = I couldn’t open [it].
• Ben açamadım. = I couldn’t open [it].
  stronger emphasis on I

The same understood subject also applies to unutunca here.

In Turkish, clauses with -ınca / -ince often leave the subject unstated when it is clear from context.

Here, the main verb açamadım clearly means I couldn’t open, so the most natural reading is that the same person also forgot/left the key inside.

So the sentence is understood as:

• When/since I left the key inside, I couldn’t open the back door.

If Turkish wanted to make the subject explicit, it could, but it usually doesn’t need to.

Because Turkish normally puts the verb near the end of the clause.

This sentence follows a very typical Turkish pattern:

• Anahtarı içeride unutunca
• arka kapıyı açamadım

Both clauses place the object before the verb:

• anahtarı before unutunca
• arka kapıyı before açamadım

This is part of Turkish’s general subject-object-verb tendency, although word order can shift for emphasis.

Not always. It often means when, but depending on context it can also feel like:

• once
• after
• since
• because

In this sentence, English speakers often prefer a causal translation:

• Because I left the key inside, I couldn’t open the back door.

But a more literal gloss with when is also possible:

• When I left the key inside, I couldn’t open the back door.

So the exact nuance comes from context.

Not very naturally. A very close structural gloss might be:

• Having forgotten the key inside, I couldn’t open the back door.
• When I forgot the key inside, I couldn’t open the back door.

But the most natural English sentence is usually:

• I couldn’t open the back door because I left the key inside.

This is a good example of how Turkish and English often package the same idea differently.

It is literally built from:

• arka = back / rear
• kapı = door

So yes, literally it is rear door/back door, but in normal English back door is the best translation.

This kind of noun phrase is very common in Turkish:

• ön kapı = front door
• arka bahçe = back garden / backyard
• arka sokak = back street