Sahi, kartı kim aldı?

Sahi is a conversational/discourse word. It often means something like come to think of it, oh right, by the way, or actually depending on context.

It does not change the basic grammar of the sentence. You can remove it and still have a complete sentence:

Kartı kim aldı?
= Who took the card?

Adding sahi just gives the sentence a more natural spoken feel.

The comma shows that sahi is being said as a separate little comment before the main sentence. In speech, there is usually a short pause:

Sahi, kartı kim aldı?

So the comma is not about case or tense; it just reflects the pause and the discourse-marker use of sahi.

The -ı is the accusative suffix. It marks kart as a specific direct object.

So:

• kart = a card / card in a more general or non-specific sense
• kartı = the card / that card as a specific object

In this sentence, the speaker is asking about a particular card, so kartı is used.

Because kim is the subject here, not the object.

Turkish often shows grammatical roles through case endings, not just word order. Here:

• kartı has the accusative ending -ı, so it is the object
• kim has no case ending here, so it is the subject

That is why the sentence means Who took the card?, not Whom did the card take?

Both are possible, but they do not sound exactly the same.

• Kim kartı aldı? = a more neutral/basic order
• Kartı kim aldı? = puts extra attention on the card

Fronting kartı makes it feel like the card is already known in the conversation, and now the important question is who took it.

So this word order is very natural in Turkish because word order is flexible and can show topic or focus.

Aldı is the simple past (also called the definite past) of almak.

Breakdown:

• verb stem: al- = take/get/buy depending on context
• past tense ending: -dı
• aldı = he/she/it took

Because the subject is kim (who), English translates it naturally as who took.

It is -dı because of vowel harmony.

The last vowel in al- is a, which is a back vowel, so the past-tense suffix appears in the back-vowel form -dı rather than -di.

As for pronunciation:

• i = the normal dotted i
• ı = the dotless i, a different vowel

The ı sound does not exist as a direct match in standard English. It is a short, relaxed back vowel said without rounding the lips.

So:

• aldi would be wrong
• aldı is correct

Because this is a wh-question with kim (who).

In Turkish, mi/mı/mu/mü is mainly used for yes/no questions, for example:

• Kartı aldı mı? = Did he/she take the card?

But with a question word like kim, you normally do not add mi:

• Kartı kim aldı? = Who took the card?

So the presence of kim already makes it a question.

Turkish does not use do-support the way English does.

In English, questions often need do/did:

• Who did take the card?
  or more naturally:
• Who took the card?

In Turkish, you do not add a separate helping verb like did. The main verb simply appears in the correct tense:

• aldı = took

The sentence becomes a question because of:

1. the question word kim
2. normal question intonation

So Turkish handles this much more directly than English.