Learn Hardcore Turkish: Kuvvetli yağmurda maratona katılmak risklidir. - It is risky to participate in a marathon in heavy rain.

It’s the locative case, meaning “in/at/on.” Turkish vowel and consonant harmony determine the exact form. Since yağmur ends in a voiced consonant and its last vowel u is a back vowel, you attach -da rather than -de.

Can you drop kuvvetli and just say yağmurda?

Yes. Kuvvetli means “strong” or “heavy.” Without it, yağmurda simply means “in the rain.” Adding kuvvetli emphasizes that the rain is heavy.

Why is maratona in the dative case with -a?

The verb katılmak (“to participate” or “to join”) requires its object in the dative case. So “to participate in a marathon” is literally maratona katılmak.

Why isn’t there an article like “a” or “the” before maratona?

Turkish has no definite or indefinite articles. You express “a” or “the” through context, not by adding a separate word.

What role does katılmak play here, and why is it in the infinitive form?

Here katılmak is a verbal noun (a gerund). In Turkish, you use the infinitive suffix -mak/-mek to turn verbs into noun phrases. That phrase, maratona katılmak, functions as the subject of the sentence.

Why do we say risklidir instead of just riskli?

Riskli means “risky.” Adding the copula suffix -dir yields risklidir = “it is risky.” In informal spoken Turkish you can drop -dir, but in standard or written Turkish it completes the predicate.

Could you use a different Turkish word for “risky,” like “dangerous”?

Yes. You can replace riskli with tehlikeli (“dangerous”) and say:
Kuvvetli yağmurda maratona katılmak tehlikelidir.

How do you make the sentence negative, to say “participating … is not risky”?

Insert değil after the adjective or use the negative copula. Either of these works:
1) Kuvvetli yağmurda maratona katılmak riskli değildir.
2) Kuvvetli yağmurda maratona katılmak riskli değil.