Modal Forms

Modal verbs can be used in a variety of different forms.

Modal Simple I could swim at the beach. Modal Continuous  I could be swimming at the beach right now. Modal Perfect  I could have swum at the beach yesterday. Modal Perfect Continuous  I could have been swimming at the beach instead of working in the office.

Passive Modal Simple  The room should be cleaned once a day. Passive Modal Continuous  The room should be being cleaned now. Passive Modal Perfect  The room should have been cleaned yesterday. Passive Modal Perfect Continuous  The room should have been being cleaned but nobody was there. (Rare form)

Source: http://www.englishpage.com/modals/modalforms.html