Climbing a rope is not as difficult as you might think.[___]

“Climbing” names the activity and functions as the subject of the sentence: the activity is not as difficult as you might think. An -ing verb form used as a noun is a gerund, so “Climbing a rope” is a gerund phrase. It is not a participle here because it does not describe a noun, and it is not an infinitive because it does not use “to” plus a verb.

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