Cevap kartını hatıra olarak sakladım.

Because cevap kartını is the definite direct object of the verb sakladım.

The base form is cevap kartı = answer card.
When a noun is a specific object in Turkish, it usually takes the accusative ending. Here that ending is -ı, but because cevap kartı already ends in a third-person possessive-style ending (-ı in the compound form), a buffer n appears before the accusative:

• cevap kartı = answer card
• cevap kartı-nı = the answer card

So cevap kartını sakladım means I kept the answer card.

Cevap kartı is a very common Turkish noun-compound pattern.

In Turkish, when one noun describes another noun, the second noun often takes a compound ending. So:

• cevap = answer
• kart = card
• cevap kartı = answer card

This pattern is similar to:

• telefon numarası = phone number
• otobüs durağı = bus stop
• yemek masası = dining table

So cevap kartı is not random; it follows a normal Turkish compound structure.

Hatıra olarak means as a souvenir, as a keepsake, or as a memento.

The word olarak often means as or in the role of. It shows how something is regarded or used.

So:

• hatıra = memory, souvenir, keepsake
• hatıra olarak = as a souvenir / as a keepsake

In this sentence, it explains why or in what sense the speaker kept the answer card.

Because hatıra is not the main direct object of the sentence.

The thing being kept is cevap kartını.
The phrase hatıra olarak is an adverbial expression meaning as a souvenir. It tells us the purpose or role of the object, not a second object.

So the structure is basically:

• cevap kartını = the answer card
• hatıra olarak = as a souvenir
• sakladım = I kept

That is why hatıra stays in its plain form.

Sakladım comes from the verb saklamak.

It can mean:

• to keep
• to save
• to put aside
• sometimes to hide, depending on context

Here, the meaning is most naturally I kept or I saved.

Its structure is:

• sakla- = verb stem
• -dı = past tense
• -m = I

So sakladım = I kept / I saved.

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

Here, sakladım already means I kept, because the ending -m marks the first person singular.

So:

• sakladım = I kept
• ben sakladım = I kept

Both are possible, but ben is usually unnecessary unless you want emphasis, contrast, or clarity.

No, Turkish word order is flexible, although this version is very natural.

The sentence:

• Cevap kartını hatıra olarak sakladım.

is a normal, neutral way to say it.

But Turkish can move parts around for emphasis. For example:

• Hatıra olarak cevap kartını sakladım.
• Cevap kartını sakladım hatıra olarak.

These are possible, though the first sentence sounds the most neutral and standard.

In Turkish, the verb often comes at the end, and the element right before the verb can get special focus.

Not necessarily. Saklamak can mean hide, but it also very often means keep or save.

For example:

• Parayı sakladım. = I hid the money / I kept the money safe.
• Mektubu sakladım. = I kept the letter.
• Bunu hatıra olarak sakladım. = I kept this as a souvenir.

In your sentence, because of hatıra olarak, the meaning is clearly kept as a keepsake, not hid.

Because Turkish endings follow vowel harmony.

The accusative ending has four forms:

• -ı
• -i
• -u
• -ü

Which one appears depends on the last vowel of the noun. In kartı, the relevant vowel pattern leads to -ı.

So:

• cevap kartı
• cevap kartını

This is part of a bigger Turkish pattern, not something special to this sentence.

It is understandable, but in natural English as a souvenir, as a keepsake, or as a memento sounds better.

That is because hatıra in this sentence does not mean a memory inside your mind. It means a physical thing you keep in order to remember something.

So the best natural translations are:

• I kept the answer card as a souvenir.
• I kept the answer card as a keepsake.
• I saved the answer card as a memento.

These fit the Turkish meaning more naturally than as a memory.