Modal verbs are conjugated in the third person singular by adding the suffix "-s," following the standard rule for regular verbs in the simple present tense in English grammar usage in formal contexts.

The modal "should" is primarily used to express a logical deduction about a past event that did not happen, functioning as a perfect synonym for "could have."

The modal verb "can" is the most formal and recommended form for asking permission in official government documents and legal contracts in the United Kingdom.

The verb "must" and "have to" are semantically identical in all contexts, both being used exclusively to express internal obligations imposed by the speaker on themselves in formal discourse.

The modal verb "might" typically expresses a lower degree of probability than "may" in formal academic discourse, while "could" can be used to make polite requests or suggest possibilities.