E. Le了at the end of a shì是sentence.
E 1.1 Xiànzài shì xiàtiān le, cài dōu piányi le.
It’s summer now, (so) vegetables have become inexpensive.
This sentence implies that vegetables were expensive before. Compare it with the following sentence which does not have the change-of-state le了.
E 1.2 Xiànzài shì xiàtiān, cài dōu hěn
It’s summer now, vegetables are inexpensive.
This sentence simply states the fact that the time is summer and that vegetables are inexpensive. It does not imply any change.
E 2.1 Tā shì lǎoshī le.
He is a teacher now. /He has become a teacher now.
This sentence shows that ‘he’ has come to a new stage of his life or a change in identity. Compare it with the following sentence which does not have the change-of-state le了.
E 2.2 Tā shì lǎoshī.
He’s a teacher.
This sentence simply announces ‘his’ identity as a teacher.