Modal adverb

The Cambridge Grammar of the English Language provides the following non-exhaustive list of modal adverbs at different levels of strength.

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

Syntax and meaning

Modal adverbs often appear as clause-initial adjuncts, and have scope over the whole clause, as in (1) with the adverb in bold.

1. Probably, the biggest push for corruption prosecutions came in the mid-2000s.

This has the same meaning as (2) with the paraphrase using the modal adjective (in bold).

1. It is probable that the biggest push for corruption prosecutions came in the mid-2000s.

Without the comma, the adverb has scope only over the NP, as in (3).

1. Probably the biggest push for corruption prosecutions came in the mid-2000s.

This can be paraphrased as (4).

1. It is probable that push for corruption prosecutions that came in the mid-2000s was the biggest such push.

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 verbs underlined.

1. That's probably going to fail.
2. That probably failed because of poor planning.
3. It could possibly help me.
4. It possibly helped me.