Gerund meaning: The gerund

The gerund , or gerund phrase, is "opening the door." It's ambiguous what it modifies. One way to clarify that would be to put a comma beforehand, making it clear it's an absolute phrase that modifies the entire sentence, thus clearly establishing the subject-verb "I saw" is being modified by the gerund , meaning "I" is who is "opening the door." Another way to clarify it would be to change "Tom" to "Tom's," the possessive case before a gerund making it adjectivally modify the gerund and thus ... It always expresses obligation: Having to ask this question again is a bummer. There it is used in a gerund phrase at the beginning of a sentence, and it is the subject. Also, in your last example, having to does not replace you have to. Having to has no subject. To replace you have to, it would be necssary to say your having to. She refused to pay the penalty. She refused paying the penalty What is the difference in meaning between these two sentences? For instance, here's one definition in a grammar usage manual (on my bookshelf): Appositive phrases are noun phrases that rename or further identify preceding nouns. -- But then that book has a "traditional grammar" type of definition for noun phrase, such that a NP can also actually be a gerund or infinitive phrase (shrugs). So, what's your grammar book say?