Given the root 'söz' (word), what is the correct form when the past tense suffix '-di' is applied, and what consonant assimilation rule is at play?

When the past tense suffix '-di' is applied to the root 'söz', the correct form is 'sözdü'. The consonant assimilation rule at play here is vowel harmony and consonant voicing. Turkish has a system of vowel harmony, meaning that vowels within a word and its suffixes tend to match each other in terms of frontness and roundedness. The root 'söz' contains back, unrounded vowels ('ö' and 'ü'). However, the consonant voicing rule is more directly relevant to the suffix change. Turkish has a rule where voiceless consonants at the end of a root word cause the following voiced consonant in a suffix to become its voiceless counterpart. The final consonant of 'söz' is 'z', which is a voiced consonant. The past tense suffix is '-di', where 'd' is a voiced consonant. Since 'z' is voiced, the 'd' in '-di' remains voiced, and no assimilation occurs based on voicing in this specific instance to change 'd' to 't'. However, if the root ended in a voiceless consonant like 't', 'p', 'ç', or 's', the suffix '-di' would change. For example, with the root 'at' (horse), the past tense is 'at tı' because 't' is voiceless and causes 'd' to become 't'. In the case of 'söz', the 'z' is voiced, so the 'd' in the suffix '-di' correctly remains 'd' after the vowel harmony has already dict....