Question

the verb 1. they will serve you lunch if you (attend/will attend) the function​

Answer 1

Answer:

They will serve you lunch if you attend the function.

Answer 2

Answer:

They will serve you lunch if you attend the function.

Explanation:  

In the given sentence, the main verb which is serve has been used in the simple ‘future sentence’. Thus the sentence is being reported in the ‘future tense’. The secondary or additional verb thus has to be in the simple present and as a result attend is the right answer.  

This is because two verbs cannot be used in the future tense in the same sentence. There is no agreement between the verb and the subject if both the verbs are in the same tense. Hence will attend is the incorrect option.