Dan-san,
actually, your last question confirmed me "what" exactly you wanted to know.
I knew it was strange to say "I can be [adjective-er]".
See, in English, it says "can run faster", so it's different from simple "[adjective]-er" you forcused on at first.
This is "can [verb] + [adjective-er]".
In such case, you have to express "can [verb]" and then, add "adjective-er". This also means that
adjectives in such sentences modify verbs. To do so, adjectives must be conjugated to adverbial form,
NOT comparative form.
=> (Watashi wa) motto hayaku hashiru koto ga dekimasu.
You are not bothering or annoying me at all
I just can't give you a wrong answer and that's why I needed
some example sentence. Now you clearly see I had a point, right?
Now I could give you a very important information on this "can do more ....ly" kind of sentences
Natsuko(奈津子),
Team JapanesePod101.com