How do you know when to use ''in'' or ''de'' with Turkish nouns?

In Turkish language, the use of ''in'' or ''de'' with nouns is determined by the vowel harmony. Vowel harmony is an important grammar rule in Turkish language that requires the last vowel of the suffix to harmonize with the vowel of the noun.

When the last vowel of the suffix is a back vowel (a, ı, o, u), the preposition ''da'' (or its shortened form ''ta'') is used. On the other hand, when the last vowel of the suffix is a front vowel (e, i, ö, ü), the preposition ''de'' (or its shortened form ''te'') is used.

For example, to say ''in Istanbul'' in Turkish, you would say ''İstanbul''da, because the last vowel of the suffix ''da'' is a back vowel which harmonizes with the back vowel ''u'' in ''Istanbul''.

Similarly, to say ''in the school'' in Turkish, you would say ''okul''da, because the last vowel of the suffix ''da'' is a back vowel which harmonizes with the back vowel ''u'' in ''okul''.

However, to say ''in the market'' in Turkish, you would say ''pazar''da, because the last vowel of the suffix ''da'' is a back vowel which harmonizes with the back vowel ''a'' in ''pazar''.

It is important to note that there are some exceptions to this rule, and the best way to learn them is through practice and exposure to the language.