This is an infinitive - to + bare infinitive. It is not an adverb as they usually end in -ly. (He drives dangerously). It is not a gerund as it ends with -ing (Reading is fun). It is not a participle because there are several kinds of participle: 1. present - Lying on her bed, she was thinking about him. 2. past - The books were on the table, covered in binders. 3. perfect - Having finished her homework, she went outside. So naturally, infinitive is the only choice.