NOT A TEACHER
IF I understand my books, here is at least one way to parse those sentences.
1. "I am very comfortable speaking French."
a. "Comfortable" is an adjective.
b. "speaking French" is a gerund phrase.
c. A gerund is, by definition, a noun.
d. Nouns sometimes actually do modify an adjective.
e. Thus, "speaking French" is a noun modifying the adjective "comfortable." So it means something like: "I am comfortable to the extent of speaking French." Compare: "I am NOT comfortable writing French."
f. Therefore, "speaking French" is an adverbial element.
2. "They had a great time swimming last Sunday."
a. "We can ... use gerunds after the object of some verbs, e.g., consider, call, declare, have, take, and spend."
b. Congratulations! You sensed a preposition in front of "swimming."
c, Actually, the preposition is "a," which means "on."
d. In much older English, your sentence might have been expressed as "They had a great time a-swimming last Sunday."
e. In modern English, one just says that the gerund phrase "swimming last Sunday" is an objective complement of the object "time." It completes the meaning of "a great time."
f. Thus, "swimming last Sunday" is a noun element.
1. Pence and Emery, A Grammar of Present-Day English (1947 and 1963), page 314.
2. Lloyd and Day, Active Grammar Level 3 (2011), courtesy of Google "books."
3. House and Harman, Descriptive English Grammar (1931 and 1950), page 316.
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