Modals are often called helping verbs, because they modify a verb, but in arguments they can undercut one's authority. This use of modals or weakens, the certainty of a sentence. The stronger the modal, the stronger the possibility.

Must is so strong that it is almost forcing something to happen. On the opposite end, can, could, and might are all equally weak and show a lack of commitment or confidence. Below is a list of modals ranked by their strength:

Weak (you disagree with this)	Strong (you are not convinced)
may can could might	no modal (active verb) will must should