Learn Turkish: Poyrazın yelkenleri şişirmesiyle tekne hızlandı. - With the northeast wind filling the sails, the boat sped up.

This word is the core of the temporal-causal link. It starts with the verbal noun şişirme (filling). Next, the third-person possessive suffix -si is added to agree with the doer of the action (the wind). Finally, the instrumental suffix -yle (with) is attached. Together, -mesiyle translates to "with its filling" or "upon its filling".

Why is poyraz (northeast wind) in the genitive case (poyrazın)?

When a noun acts as the subject or "doer" of a verbal noun like şişirmesiyle, it must take the genitive case. Turkish treats this action as a possession: literally, it translates to "With the northeast wind's filling of the sails..."

Does şişirmesiyle express time (when) or cause (because) here?

It expresses both simultaneously, which is the beauty of the -mAsIylA structure! It implies that the boat sped up at the exact moment the sails filled (temporal), and it sped up because they filled (causal).

How is the verb hızlandı formed?

It is built from the noun hız (speed). The suffix -lan is commonly used to turn a noun into an intransitive verb, creating hızlanmak (to gain speed, to accelerate). The -dı at the end is simply the definite past tense marker.