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describe the grammatical function of the verb 'to be':
+/ He is a world famous singer.
+/ She has been in London at least twice.
+/Her letters were not found anywhere.
+/ They are sitting side by side in the dark.
+/ He is to go to the station at once if he doesn't want to be late.
+/ 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.
describe the grammatical function of the verb 'to be':
+/ He is a world famous singer.
+/ She has been in London at least twice.
+/Her letters were not found anywhere.
+/ They are sitting side by side in the dark.
+/ He is to go to the station at once if he doesn't want to be late.
The verb "to be" serves several functions depending on what form it takes.
Quote:
He is a world famous singer.
In that sentence, "world famous singer" describes "He". The verb "is" chiefly acts as a connecting word.
Quote:
She has been in London at least twice.
In that sentence, "has been" describes a state of being. Where has she been? She has been to London. (Normally, "to" is the preposition used.)
'to be' 
Linear Mode
