Modal verbs add information about mood. A writer or speaker communicates her mood, feelings or attitudes through modal verbs. In chapters 2 and 3, modal verbs such as MAY or MUST show different levels of intensity in requests, permissions, prohibitions, or obligations. Modal verbs are also useful for giving advice.
The modal verb SHOULD communicates a speaker’s strong advice. SHOULD is quite inflexible. COULD is weaker. COULD communicates possibility in addition to advice, so there is more flexibility about following the advice. MIGHT, like COULD, gives a listener the choice to follow the advice or not.
Example
Drivers should avoid distractions while they are driving. They should keep their eyes on the road. They should control the steering wheel with both hands. They should not text while driving. To avoid being distracted, drivers could turn off their cellphones. They might put their cellphones out of reach.