What are gerunds, infinitives, and participials ?

A Gerund/ Gerund Phrase is a verb form that acts like a noun and ALWAYS ends in -ing.   Hint:  Replace the gerund with "this thing," or, "that thing" to test your answer.

A participle / participle phrase is a verb that functions in a sentence as an adjective. It described nouns and pronouns.  
Present -> ends in -ing
Past ->    ends in - ed
Irregular -> various endings

Infinitive / infinitive phrases is a verb form preceded by the word "to" 

Don't confuse with the preposition "to"   
  
                              Infinitive = to + verb 
                             Preposition = to + noun or pronoun
sometimes the "to" is dropped. 

I hope this helped. Of course, this is the 11th-grade versions of English. Also, if you have any other questions let me know :)