Answer:
Parallelism has to do with the grammatical structure and cohesion of text. In order to do so, verbs, adverbs, adjectives, etc. must be paired so as to make the text more understandable and pleasing to the eyes.
1. It is harder to tie a slip knot than tying a square knot.
In this sentence, there are two tenses of verbs when there should be only one. Thus,
"It is harder to tie a slip knot than to tie a square knot."
2. We enjoyed water skiing much more than when we swam in the lake.
Another tip on correcting faulty parallelism is to coordinate the tense of the paired verb ('water + skiing') with the other pairs ('swimming + lake'). Thus,
"We enjoyed water skiing much more than swimming in the lake."
3. Driving will get you there more quickly than to walk.
Since the first tense introduced is in '-ing' form, the next verbs must also be transformed into '-ing' forms. Thus,
"Driving will get you there more quickly than walking."
