Basic principles:

Put simply, the following tables contain parts of speech put in sentences that can substitute modal verbs. Our focus in this post, which is highly useful for academic writing, would be on possibility, obligation and necessity. For example, modal verbs representing possibility (must, should, may, can, etc.) can be replaced by similar meanings in different grammatical forms such as adjectives (certain, sure, probable, possible, likely) and their corresponding adverbs (certainly, surely, of course, probably, possibly, perhaps, maybe), or even verbs (assert, advocate, believe, think, suppose, suggest, guess, assume) and nouns (certainty, probability, likelihood, assertion, assumption, and… possibility of course). This means, for the same degree of modality, you have at least four different sentence structures, based on the part of speech you use.

Let’s start with possibility first:

Various structures	Example for expressing high degree of possibility (E.g. Eric must be crazy.)
Structure A: It + linking verb + adjective + to-infinitives/ that-clause	It is certain that Eric is crazy.
Structure B: Adverb + clause	Certainly/Surely, Eric is crazy…
Structure C: Pronoun + verb suggesting modality + that-clause	I assert that Eric is crazy.
Alternative Structure C: Pronoun + passive verb form + to-infinitive.	Eric is confirmed to be crazy.
Structure D: The + noun phrase including modality + complement	The possibility of Eric being crazy is high.

It is, therefore, not difficult to apply the above structures to express obligation in various ways (must, should, may, can, etc.). For example, we can use adjectives (imperative, obligatory, be obliged to), passive verb forms (expected, supposed, required, advised, invited, suggested, recommended, etc.) and nouns (must [n], requirement, obligation, advice, invitation, suggestion, recommendation, etc.). So far there are no adverbs corresponding modals of obligation (there is one, “obligatorily”, but I aren’t sure how to put it in the sentence to mean similarly to the example):

Various structures	Example for expressing high degree of possibility (E.g. Eric must finish his tasks on time.)
Structure A: It + linking verb + adjective + to-infinitives/ that-clause	It is imperative for Eric to finish his tasks on time.
Structure B: Adverb + clause	Eric is to finish his tasks obligatorily.
Structure C: Subject + verb suggesting modality + to-infinitive	Eric is required to finish his tasks on time.
Structure C alternative: Pronoun + verb suggesting modality + that-clause	I urge that Eric finishes his tasks on time.
Structure D: The + noun phrase including modality + complement	The obligation for Eric to finish his tasks on time is high.
Structure D alternative: It + linking verb + noun phrase + complement	It is an obligation for Eric to finish his tasks on time.

What about modality that expresses necessity? I’ve just said “necessity”; of course, this word can be used to replace “need” with an adjective (necessary), adverb (necessarily) and verb (necessitate):

Various structures	Example for expressing high degree of possibility (E.g. Eric need pay his bills.)
Structure A: It + linking verb + adjective + to-infinitives/ that-clause	It is necessary for Eric to pay his bills.
Structure B: Adverb + clause	Eric necessarily pays his bills.
Structure C: Pronoun + verb suggesting modality + to-infinitive	Eric is necessitated to finish his tasks on time.
Structure D: The + noun phrase including modality + complement	The necessity for Eric to finish his tasks on time is imminent/high.
Structure D alternative: It + linking verb + noun phrase + complement	It is a necessity for Eric to finish his tasks on time.

We have come across the variations of expressing the same modality by replacing the modal verbs with other parts of speech. This causes changes in the sentence structure too. In academic writing, which structures would be more commonly used to express modality? Why some are preferred while some are not? Let’s explore these questions in our next post.

Coming up:

After this post, you may wonder, “are there subtle differences among these expressions?” The answer is affirmative, but their differences are not merely those in the sentence structure and parts of speech. In the next post, I will try illustrating the reasons motivating us “paraphrasing” modal verbs, especially in academic writing.