Substitute que
There's a way to use the subordinating conjunction que that isn't often taught in school: as a substitute for another conjunction. This usage is neither informal nor unusual, but it is limited to a specific construction and for a specific reason. 

The French expression c'est-à-dire is essential - that is, very useful. 



The rhythm of French is what makes it such a beautiful language. Thanks to liaisons and enchaînement, French words flow one into the next, and there is no stress on any particular syllable within each word. All of this combines to give French a certain musicality that sets it apart from other languages.
In English, two negatives are said to make a positive: that is, they cancel one another out, and this is grammatically unacceptable. In French, however, négation double is alive and well. Two negatives sometimes make a positive, while other times they combine to make the negation stronger or more specific. 



The best-known passive construction is the passive voice, which has a verb performing on a subject (e.g., he is seen). But did you know that French has several other passive constructions that are just as important to recognize and use? 