The Great Vowel Shift CLOZE

Target Language or Knowledge:

People who are learning English often find it difficult to pronounce English vowels. Some English vowels can be pronounced in several different ways, depending on the spelling of the word, and sometimes they are even pronounced differently in words that have similar spellings. For example, the ‘ea’ in ‘bread’ is pronounced the same as the ‘e’ in ‘bred,’ and not the same as the ‘ea’ in ‘break.’ English pronunciations weren’t always so difficult. In the past, English vowels were pronounced the same as vowels in formal Spanish. There was only one way to pronounce each vowel. Over a period of 350 years, from the fourteenth to the eighteenth centuries, English pronunciations changed drastically. People who study languages refer to this as “The Great Vowel Shift.”

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What is the "Great Vowel Shift: and what problems does it cause?