What is the infinitive in the sentence 'to hurl stones at flowers is silly.'

to hurl

Explanation:

Infinitives are verbals which are the words made from verbs but functioning as something else, noun, adjective, adverb etc.). The verbals are of three types infinitives (to hurl a stone, to fly is interesting), gerunds (smoking kills), participles (he is a learned man, The dead plants).

Verbals made by adding "to" before first form of verb are called infinitives e. g To swim in the river requires skill, He like to study at night.

In the sentence provided "to hurl stones" is a noun phrase, acting as direct subject.