In the first sentence “Filling “is a GERUND. The gerund has the function of a noun and in this particular example it is part of the subject of the sentence. In the last sentence “to run” and “to walk” is an INFINITIVE. Finally, in the third one, “riding” is a PARTICIPLE. A participle has the function of an adjective and in this particular example it is part of a participle clause.
Yes with having a friend by your side you can overcome everything.so it’s both.
The letter was posted by his father.
