because people speak English
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Syntax is the grammatical tool that deals with how sentences are put together and the relationship between words. It is a very methodical and logical sequence, ensuring that sentences are put together using subject, verb and object and that the words in the sentence all have agreement, so that the correct forms of words are used.
Without syntax to structure the language, it would simply be a string of words that makes no sense.
For example, correct syntax would state that, “The boy loves to eat sweets”. This follows the syntax of subject, verb, object and agreement. Without syntax, we could simply write, “Sweets loves to eat the boy”. These are the same words, but they have a totally different meaning. http://www.grammartips.net/syntax-in-english-grammar