The document discusses modal verbs used for deduction and possibility in the English language. It explains that modal verbs like "must", "may", and "might" are used to make deductions or suppose possibilities, rather than stating known facts. The choice of modal verb indicates the level of certainty in the deduction or possibility. It also discusses the continuous and perfect infinitives used with modal verbs of deduction, such as "must be working" or "may have spoken".