Clauses are groups of words which have meaning, but are often not complete sentences. Some clauses begin with the word "when" such as "when she called" or "when it bit me." Other clauses begin with "while" such as "while she was sleeping" and "while he was surfing." When you talk about things in the past, "when" is most often followed by the verb tense Simple Past, whereas "while" is usually followed by Past Continuous. "While" expresses the idea of "during that time." Study the examples below. They have similar meanings, but they emphasize different parts of the sentence.