Modal verbs are a special type of auxiliary verb that are used with the infinitive form of the main verb to express ideas like certainty, obligation, permission, or ability. The most common modal verbs are will, shall, can, could, may, might, should, would, and must. Modal verbs always take the infinitive form of the main verb and do not change based on grammatical person. They can be used to express degrees of certainty, ask or give permission, or make suggestions.