2.3.3 Progressive with adverbs or indefinite frequency

Rule: The progressive can be used to express a temporary habit, but not with an adverb of indefinite frequency.

Examples of errors:

1. Today we have no opportunity to ride horseback, so we only go for walks, and sometimes we are swimming.
2. Sometimes he is sitting in his room the whole day reading a book.
3. Electric cars are not as noisy as ordinary cars, which often are disturbing people, especially in big cities.

The usage referred to in Rule 2.3.3 (of. Leech 1971:§49; Edmondson et al. 1977:127) normally requires an adverbial of definite time (these days, this winter, etc.), which specifies the limits of the duration of the activity and thus marks it as temporary, e.g.:

We're swimming a lot these days.
He's sitting in his room a lot this semester.
Electric cars are disturbing people more often than we thought they would

Such sentences emphasize the temporary nature, of a series of events or actions, i.e. a temporary habitual occurrence. An adverbial of indefinite time, like sometimes or often, on the other hand, implies indefinite rather than limited duration and contradicts the notion of the progressive as an indicator of temporary activity. (More often than we thought can be considered definite or specific, as opposed to often alone.)