This question tests your understanding of modal verbs, which are used to express necessity, obligation, advice, or possibility. The correct choice depends on the context and the strength of the advice or instruction being given.

Let's analyze the sentence: "Oh, you are seriously ill! I think you ______ consult a doctor and if he tells you to keep to bed you _____ do that."

Step 1: Analyzing the first blank
The speaker is giving strong advice to someone who is "seriously ill". The most appropriate modal verb for giving strong advice or expressing a strong recommendation is "should". "Must" is also grammatically correct but is stronger, implying an obligation or necessity rather than advice. In this context of friendly concern, "should" is the most natural fit.

Step 2: Analyzing the second blank
The second part of the sentence describes a consequence: "if he tells you to keep to bed...". A doctor's advice, especially for a serious illness, is an instruction that must be followed. Therefore, the blank requires a modal verb that expresses strong obligation or necessity. "Must" is the perfect fit here, as it conveys that following the doctor's orders is not optional.

Step 3: Evaluating the options
Let's look at the pairs:

• should/must: This fits our analysis perfectly. Strong advice ("should") to see a doctor, and strong obligation ("must") to follow the doctor's orders.
• has to/may: "has to" is acceptable for the first blank but is less common in this advisory context. "May" expresses permission, which is too weak for a doctor's instruction.
• must/can't: "Must" is acceptable for the first blank, but "can't" (cannot) is the opposite of what is needed; it means you are unable to follow the orders.
• have to/couldn't: "Have to" is similar to "must" and could work for the first blank. However, "couldn't" (could not) is also the opposite of the required meaning, suggesting an inability to follow the doctor's advice in the past.

Final Answer: The most appropriate option is should/must.

Related Topic: Modal Verbs

Modal verbs are auxiliary verbs that express ability, possibility, permission, obligation, or advice. The key modal verbs are:

• Should: Used to give advice or make a recommendation.
• Must: Used to express strong obligation or necessity (from the speaker's perspective).
• Have to: Also expresses necessity, often due to an external rule or circumstance.
• May/Could: Used to express possibility or permission.
• Can't: Expresses inability or impossibility.

Choosing the right one depends on the context and the degree of certainty or obligation you want to convey.

Key Formulae/Theory

Subject + Modal Verb + Base Form of Verb
This is the fundamental structure for using modal verbs. The modal verb (e.g., should, must) never changes form for the subject (e.g., I should, he should, they should). It is always followed directly by the base form of the main verb (e.g., go, eat, consult).

Example: $\mathrm{You} \mathrm{should} \mathrm{consult} a \mathrm{doctor}.$