Learn Turkish: Zayıf adam o soğuk suda daha fazla çırpınamazdı. - The weak man couldn't have struggled any more in that cold water.

It breaks down as çırpın (struggle) + -a (inability) + -maz (aorist negative) + -dı (past tense). Using the aorist past with the inability marker expresses a negative counterfactual: an action that was physically or logically impossible in that past situation.

Does çırpınmak mean 'to struggle' in a general sense, like struggling with a math problem?

No, çırpınmak refers to physical thrashing, fluttering, or frantic movement. It comes from the verb çırpmak (to beat or flap) with the reflexive suffix -ın, literally meaning 'to flap oneself around.' For a drowning person or a trapped bird, it is the exact right word. For a non-physical struggle, you would use a verb like zorlanmak instead.

Why doesn't su take a buffer letter -n- before the locative suffix, making it suyunda?

Because soğuk su (cold water) is just an adjective followed by a noun, not a noun compound. Buffer letters like -n- are used when a noun has a possessive suffix, like in the noun compound deniz suyunda (in the sea water). Since there is no possessive suffix here, we just attach -da directly to get suda.

In English we say 'any more,' but daha fazla usually means 'more.' Is this the standard way to say 'any more' in negative sentences?