Bob was really happy!He ... an important contract. 1) was signing

Bob was really happy! He had signed an important contract. Боб был вправду счастлив! Он подписал важный договор.

Here we use Past Perfect (had signed). The past perfect refers to a time earlier than before now. It is used to make it clear that one event happened before another in the past.

Past Perfect is formed with the help of the second form of the auxiliary verb to have had and with the help of the third form of verb.

Negative is formed with the help of not which stands after the auxiliary.

He had not signed an important contract.

Interrogative Had + subject + the third form of the verb.

Had he signed an important contract?