Learn Dutch: Het grote feest zal een succes worden. - The big party will become a success.

In Dutch, when you use a helping verb like zal (will) to talk about the future, the main verb—in this case worden (to become)—is pushed all the way to the end of the sentence in its full dictionary form (the infinitive).

Why do we use zal here instead of zullen?

Zullen is the plural form. Because the subject of this sentence, het grote feest (the big party), is a singular "it", we must use the singular form zal.

Feest is a het-word. Why does the adjective get an -e at the end to make it het grote feest?

For het-words, you only drop the -e from the adjective if you use een (a) or no article at all, like in een groot feest. Because this sentence uses the specific article het (the), the adjective gets the standard -e ending. Notice that the spelling also changes from groot to grote to keep the 'o' sounding long!

Could I say zal een succes zijn instead of zal een succes worden?

Yes, absolutely! ...zal een succes zijn means "will be a success." Using worden (to become) just sounds a bit more dynamic to a Dutch speaker, emphasizing that the event will turn out to be a success. Both are perfectly correct.