Learn Turkish: Casus teşkilata ihanet ediyor. - The spy is betraying the agency.

In English, you betray someone (using a direct object). However, the Turkish compound verb ihanet etmek translates more closely to "to commit betrayal". Therefore, you commit betrayal to someone or something, which requires the dative case -(y)a / -(y)e.

Why doesn't the 't' at the end of teşkilat change to a 'd' before the vowel -a?

Usually, a hard t at the end of a word softens to a d when a vowel is added. However, teşkilat (agency/organization) is an Arabic loanword, and many of these loanwords are exceptions to the consonant mutation rule. The 't' remains hard, so it stays teşkilata.

If teşkilat doesn't mutate, why does the 't' in etmek change to a 'd' in ediyor?

Unlike the loanword teşkilat, the native helper verb etmek is one of a few common words that always follow the consonant mutation rule. When the vowel of the continuous suffix -iyor attaches to it, the t softens to d. Seeing teşkilata and ediyor right next to each other is a great reminder of how these rules apply differently to different words.

How do we know casus means "the spy" and not "a spy"?