What is going to happen? Put the verbs in brackets in the correct form and write the sentence down. My classmates decided to arrange a party. They (have) a nice time.

The sentence describes a past decision and a subsequent event. The verb 'have' should be in the past tense to match the context of the decision made in the past. Therefore, the correct form is 'had'.

The corrected sentence is: My classmates decided to arrange a party. They had a nice time.