Sentence:  Most of these embellishments are played very quickly, in order that the note, before which they are placed, and to which they are added as <WSD>grace notes</WSD>, may lose but little of its value. It is often difficult to guess from which note (whether the preceding or succeeding) the time requisite for the execution of the <WSD>grace note</WSD> is to be taken. 
Meaning of the word marked with <WSD> tags:  A musical note, indicated on a score in smaller type with or without a slash through it, played to ornament the melody rather than as part of it. Its note value does not count as part of the total time value of the measure it appears in.