Learn Turkish: Bölgesel krizleri önleyebildiler. - They were able to prevent the regional crises.

It attaches to the noun bölge (region) to create the adjective bölgesel (regional). The -sAl / -sEl suffix is frequently used in modern Turkish to turn nouns into "relational adjectives" (meaning "related to X"), very much like the English suffixes "-al" or "-ic".

Why does krizleri have the -i suffix at the end?

The -i is the specific accusative case suffix. Because the English translation specifies the regional crises (a specific, known set of crises rather than crises in general), it must take the accusative ending as the direct object of the verb.

What are the exact suffixes attached to the verb in önleyebildiler?

It breaks down like this: önle- (verb root: to prevent) + -y- (buffer consonant) + -ebil- (ability suffix: to be able to) + -di (definite past tense) + -ler (3rd person plural: they). Altogether, it means "they were able to prevent".

Is there a connection between the word ön (front) and the verb önlemek?

Yes! The noun ön means "front" or "before." Adding the verbalizing suffix -le creates önlemek, which literally comes from the idea of "getting in front of" something or heading it off, giving us the meaning "to prevent."