"türkçe dersi" is a Genitive construction, meaning that the relevant phrase consists of two nouns that are related to each other in terms of possession and connection (türkçe ders-i) When setting a sentence; - if "türkçe dersi" is in nominative case, there is not any manipulations on the phrase. Example: Türkçe dersi, yabancılar tarafından sevilen bir ders. (Turkish lesson is a beloved lesson by foreigners.) -if "türkçe dersi" is in dative case, it receives "-e" case ending to become the indirect object of a verb. In this case, "-e" comes after "i", hence "n" added between them to avoid pronunciation difficulties: dersi + (n)e Example: Dün Türkçe dersine katılamadım. (I couldn't attend to Turkish lesson yesterday.)

Türkçe dersi=>Turkish lesson Türkçe dersine=>for Turkish lesson(class)

Türkçe dersi zor. Özne----Turkish lesson is difficult. Türkçe dersine geç kaldım. Nesne---- I am late for Turkish lesson.