+/ He is a world famous singer. copula verb
+/ She has been in London at least twice. past participle with a meaning similar to 'go'
+/Her letters were not found anywhere. auxiliary verb to form the passive
+/ they are sitting side by side in the dark. auxiliary verb to form the present progressive (continuous)
+/ He is to go to the station at once if he doesn't want to be late. functioning like a modal, with a sense of obligation like must.