Learn Turkish: Kimyasallı su içmesi, midesinde enfeksiyon oluşmasına sebep oldu. - His drinking water containing chemicals caused an infection to form in his stomach.

The verb sebep olmak (to cause) requires the dative case for the outcome it causes. Here, the outcome is the verbal noun phrase enfeksiyon oluşması (the forming of an infection). To link this phrase to the main verb sebep oldu, we add the buffer -n- and the dative -a, resulting in oluşmasına.

Both içmesi and oluşması end in the possessive suffix -si. Who or what is the possessor in each case?

For içmesi, the implied possessor is onun (his/her), making it 'his drinking'. For oluşması, the grammatical 'possessor' is the subject of the action: the infection. In Turkish, verbal nouns (actions turned into nouns with -mA) take possessive endings to show who or what is doing the action. Thus, enfeksiyon oluşması functions structurally like 'the infection's forming'.

Why is it just enfeksiyon oluşmasına instead of enfeksiyonun oluşmasına with the genitive case?

When the subject of a verbal noun clause is indefinite or general (an infection, rather than a specific known infection), Turkish often drops the genitive suffix. Enfeksiyon oluşması acts like a general noun compound ('infection formation'). If you used enfeksiyonun, it would imply the formation of one specific, previously mentioned infection.

What is the exact function of the -lı suffix in kimyasallı?