1) what is the main verb in this sentence?

"I bet you could do it if your life depended on it"

2) what is the main verb in the following sent?

"The coffee mug that I drink out of every morning has gone missing"

The main sentence for question number 1 is do and for question number 2 the answer is gone

The main verbs in the given sentences above are the following:

1. bet - The subject is the word "I" and the direct object of this verb is the word "you".

2. gone missing - The subject is the word "coffee mug". The phrase "that I drink out of every morning" is just a phrase that describes about the coffee mug, that's why it is not considered as the main verb of the sentence. This part can be omitted without affecting the whole thought of the sentence.