How do you identify the speakers purpose

Listen!
identify supporting details and evidence, determine and identify central ideas and arguments within the speech, listen or determine tone of voice. consider what the purpose could be

In the sentence represented above - i by preparing dinner there is a ferund phrase - preparing dinner. the only thing you need to indicate the gerund phrase is to find the world with -ing and object supported.
what about noun function in the sentence, the answer is subject complement.