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There are many words in the English language.You will never know the meaning of every word in English.When you read,you will often find many words you do not know.You will not have enough time to stop reading or try to find every new word in the dictionary.
Sometimes you can guess a new word because you know some of the parts of the new word.For example,if a word ends by the letter "er”,that word might be the name of a person or a thing that does a certain action.A writer is a person who writes.Sometimes it is not useful to know the parts of a new word to understand it,but it will help you many times.(如何翻译，大概意思）