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 dictated the choice of 'd' over 't' (though in this case, the vowel harmony would favor '-dü' if it were a simple vowel harmony rule application, but the suffix attaches as '-di' and undergoes voicing assimilation if applicable, which it is not here). The vowel 'ü' in 'söz' is a high back unrounded vowel. The suffix vowel 'i' is a high front unrounded vowel. The suffix '-di' assimilates to the vowel harmony of the root. However, the question asks about the consonant assimilation rule. The primary consideration here is how the final consonant of the root interacts with the initial consonant of the suffix. The consonant 'z' in 'söz' is a voiced velar fricative. The consonant 'd' in '-di' is a voiced alveolar stop. Turkish consonant assimilation rules primarily involve the change of voiced consonants to voiceless ones (devoicing) when they follow a voiceless consonant, or the change of voiceless consonants to voiced ones (voicing) when they follow a voiced consonant. In the case of 'söz' + '-di', the 'z' is voiced and the 'd' is voiced. Therefore, no voicing assimilation occurs that would change the 'd' to a 't'. The suffix remains '-di' and the resulting word is 'sözdü' where the final vowel of the suffix has harmonized with the final vowel of the root. The suffix is '-di' and its vowel is 'i' which is not harmonizing with the final vowel of 'söz' which is 'ü'. The correct suffix application in Turkish is not solely based on consonant assimilation but also on vowel harmony. The past tense suffix in Turkish has four forms: -dı, -di, -du, -dü. The choice depends on the final vowel of the root. 'Söz' ends in 'ö', which is a rounded back vowel. The closest matching suffix vowel is 'ü' from '-dü'. However, the rule is that the suffix vowel must harmonize with the *last vowel* of the root. The last vowel of 'söz' is 'ö'. Thus, the suffix should be '-dü'. So, 'söz' + '-dü' would lead to 'sözdü'. This is the correct form. The consonant assimilation rule is consonant voicing. If the final consonant of the root is voiceless, the initial consonant of the suffix changes from voiced to voiceless. Examples: 'gitmek' (to go) -> 'gitti' (he/she/it went) because 't' is voiceless. 'yapmak' (to do) -> 'yaptı' (he/she/it did) because 'p' is voiceless. If the final consonant of the root is voiced, the initial consonant of the suffix remains voiced. In 'söz', the final consonant is 'z', which is voiced. Therefore, the 'd' in '-di' (or more accurately, the chosen vowel-harmonized suffix, which would be '-dü') remains 'd'. Hence, 'söz' + '-dü' becomes 'sözdü'. The initial thought that '-di' is applied directly is incomplete, as vowel harmony dictates the form of the suffix. The correct form is 'sözdü'. The relevant consonant assimilation rule is consonant voicing, where the voiced 'z' at the end of 'söz' causes the initial 'd' of the past tense suffix (which, after vowel harmony, is '-dü') to remain voiced. If 'söz' ended in a voiceless consonant, the 'd' would change to 't'.