Ben anahtarı kaybettiğimi sanınca panik oldum.

Kaybettiğimi is the part that means that I lost or more literally my having lost.

It breaks down like this:

• kaybet- = lose
• -dik = a suffix that turns a verb into a noun-like clause
• -im = marks the subject of that clause as I
• -i = accusative case, because the whole clause is the object of sanmak

So:

• kaybettiğim = that I lost
• kaybettiğimi = that I lost as the thing being thought

A very literal gloss would be something like:

• anahtarı kaybettiğimi sanınca = when I thought [that I had lost the key]

Because anahtarı is a definite direct object.

• anahtar = key
• anahtarı = the key / his-her-its key / a specific key as object

In this sentence, the speaker is talking about a specific key, not just any key in general, so Turkish uses the accusative ending.

Compare:

• anahtar kaybetmek = to lose a key / keys in general
• anahtarı kaybetmek = to lose the key / a specific key

Here the meaning is clearly specific, so anahtarı is natural.

They are doing two different jobs.

• anahtarı is the object of kaybetmek
• kaybettiğimi is the object of sanmak

So inside the sentence, you have a clause inside another clause:

• anahtarı kaybettiğimi = that I lost the key
• This whole unit is what is being thought

So the structure is roughly:

• I thought [that I lost the key]

That is why the embedded clause itself gets accusative marking.

Sanınca means when one thinks/thought, upon thinking, or when I thought in this context.

It comes from:

• sanmak = to think, suppose, assume
• -ınca / -ince = when, once, upon

So:

• sanınca = when [someone] thought/supposed

In this sentence, the subject is understood to be I, because of the overall context:

• Ben ... panik oldum = I ... panicked

So sanınca here is naturally understood as when I thought.

Because sanmak often means to think/suppose/believe, especially when it is an assumption and may turn out to be wrong.

That fits this sentence well:

• the speaker believed they had lost the key
• that belief caused panic
• it may or may not have been true

Düşünmek is a broader verb meaning to think, to reflect, to consider.
Sanmak is often better for I thought X was the case.

So here, sanınca sounds very natural.

No. Turkish often leaves pronouns out when the verb already shows the person.

You could say:

• Anahtarı kaybettiğimi sanınca panik oldum.

and it would still clearly mean I panicked when I thought I had lost the key.

Ben is added for emphasis, contrast, or clarity. It can suggest something like:

• I panicked
• As for me, I panicked

So it is optional, not required.

Turkish very often uses noun/adjective + olmak to express a change of state.

So:

• panik olmak = to panic / to become panicked
• panik oldum = I panicked

This is a very common and natural way to say it.

You may also hear forms based on a verb like paniklemek, but panik oldum is extremely normal and idiomatic.

Because Turkish usually puts:

1. subordinate clauses before the verb they connect to
2. the main finite verb near the end of the sentence

So the structure is:

• Ben = I
• anahtarı kaybettiğimi = that I lost the key
• sanınca = when I thought
• panik oldum = I panicked

A literal order in English would be something like:

• I, that I had lost the key when I thought, panicked

That sounds strange in English, but it is normal Turkish word order.

Not necessarily.

The sentence only tells us that the speaker thought they had lost the key, and then panicked.

Because it uses sanmak, there is often an idea that the belief may have been mistaken. So the real situation could be either:

• the key really was lost
• the speaker only believed it was lost, but it was not

The sentence itself does not confirm which one is true.

Because Turkish and English package time differently here.

• sanınca does not work exactly like one English tense
• it means something like when, once, or upon
• the exact English tense depends on context

So anahtarı kaybettiğimi sanınca panik oldum can be translated naturally as:

• When I thought I had lost the key, I panicked.

English uses had lost because that sounds most natural: the losing is understood as earlier than the moment of thinking.

Turkish does not need to match that in the same way. The structure is more about the relationship between the events than about copying English tense choices.

Because in this kind of Turkish embedded clause, the subject is shown by a possessive-style ending on the verbal noun.

So in:

• kaybettiğim = that I lost

the -im marks the subject as I.

This is normal Turkish grammar. Even if Ben appears separately in the sentence, the embedded clause still needs its own subject marking.

That is why Turkish can pack a lot of information into one word:

• kaybettiğimi = that I lost
  • object marking for the whole clause