Final answer:
The correct modal to use in the sentence 'She got up at five today. She....be tired.' is 'must', which expresses a high degree of certainty that she is tired based on her early start to the day. Modals like 'must' are important in conveying different shades of meaning such as necessity, likelihood, or ability in English grammar.
Step-by-step explanation:
The sentence 'She got up at five today. She must be tired.' employs a modal to express assumption or logical deduction based on the information that the subject woke up early. In this case, the modal must is used to suggest that it is very likely she is tired due to the early start to her day. Modals are auxiliary verbs that express necessity, possibility, permission, or ability, and they are a fundamental part of English grammar. Examples include can, could, may, might, must, shall, should, will, and would.
In our presented contexts, characters exhibit clear signs of tiredness or lethargy, as in the reference to the character sinking into a chair due to physical exhaustion or another character feeling very tired and wanting to sleep immediately. The usage of modals can help in describing these states effectively by providing subtle nuances to the degree of certainty regarding their condition.