An infinitive without a 'to' attached is called a bare infinitive.

The verb 'help' takes to-infinitives and bare infinitives but bare infinitives are said to be the most common in casual text; as also used in this example sentence.

The Greeks made theoretical models of geometry.

It is not known when the Hinamatsuri first started, but the Nagashibina, in its most primitive form, was already being celebrated during the Heian period.