Learn Turkish: Siyah bulutlardan dolayı sağanak başlayacağını anladık. - We understood that a downpour will start because of the black clouds.

It uses the future object clause structure. It is built from başla (start) + -y- (buffer) + -acak (future suffix, softened to -acağ-) + -ı (3rd person possessive, representing "its") + -n- (buffer) + -ı (accusative case). The accusative is required because this entire future action is the direct object of the main verb anladık (we understood).

Why is the subject of the clause just sağanak instead of sağanağın? Don't these clauses usually need the genitive case?

Typically, the subject of an object clause takes the genitive case, like onun başlayacağını (that it will start). However, when the subject is indefinite or general, like "a downpour" rather than "the downpour", Turkish often leaves it in the bare dictionary form.

Why does bulutlardan have the ablative -dan ending here?

The word dolayı (because of / due to) is a postposition that always forces the noun before it to take the ablative case (-dan / -den). Therefore, "because of the clouds" must be translated as bulutlardan dolayı.

Since anladık (we understood) is in the past tense, why doesn't the Turkish sentence use a past tense for "would start"?