Modal adverbs are adverbs, such as probably, necessarily, and possibly that express modality, i.e., possibility, necessity, or contingency.[1] [2]
The Cambridge Grammar of the English Language provides the following non-exhaustive list of modal adverbs at different levels of strength.[3]
Strong: assuredly, certainly, clearly, definitely, incontestably, indubitably, ineluctably, inescapably, manifestly, necessarily, obviously, patently, plainly, surely, truly, unarguably, unavoidably, undeniably, undoubtedly, unquestionably
Quasi-strong: apparently, doubtless, evidently, presumably, seemingly
Medium: arguably, likely, probably
Weak: conceivably, maybe, perhaps, possibly
Modal adverbs often appear as clause-initial adjuncts, and have scope over the whole clause,[4] as in (1) with the adverb in bold.
This has the same meaning as (2) with the paraphrase using the modal adjective (in bold).
Without the comma, the adverb has scope only over the NP only, as in (3).
This can be paraphrased as (4).
There is a tendency for modal adverbs to follow auxiliary verbs but precede lexical verbs, as shown in (5–8) with the adverbs in bold and the verb underlined.