【Only】 ,  【alone】  are often used interchangeably (though  【alone】  is not found in the attributive position), but seldom without a slight change in meaning or emphasis.

【Only】  is especially appropriate when restriction to what is specified or asserted is implied and the term is equivalent to sole or solely; thus, "I want  【only】  this book" implies a wish for one and no more; "of all the family  【only】 John and Helen came," that is, the specified persons and no more.

【Alone】  may be chosen when the idea of the elimination of all other possibilities is expressed and the term is the equivalent of exclusive or, more often, exclusively; thus, "I want this book  【alone】  " implies a wish for a particular one and no other; "of all the family John  【alone】 came," that is, John and none of the others.

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