How French has influenced English
On December 25, 1066, William the Conqueror (also Duke of Normandy) became King of England. As a result, French became the language of the court, administration and culture and English continued to be used by ordinary people. More than 10,000 French words found their way into English – words associated with government, law, art, literature, food, and many other aspects of life. About three quarters of these words are still used, and words derived directly or indirectly from French now account for more than a third of English vocabulary. In fact English speakers know around 15,000 French words, even before they start learning the language.
You can find some French words and expressions used in the English language on the website About.com.
Watch out for (= attention à) false cognates (= faux amis)! 
photo credit: dontcallmeikke
On the website, you can also find English expressions with the word French. Here is a little selection:
French door – la porte-fenêtre
literally, window-door
photo credit: trekkyandy
French dressing – la vinaigrette
Only in England does French dressing mean vinaigrette. In the US, French dressing refers to a sweet, tomato-based salad dressing that does not, as far as I know, exist in France.
photo credit: spike55151
French fry – la (pomme de terre) frite
literally, fried potato. Note that French fries are actually Belgian
to take French leave – filer à l’anglaise (informal)
literally, to split/take off the English way
photo credit: pacomexico
French poodle – un caniche
literally, poodle
French toast – le pain perdu
literally, lost bread
Pardon my French. – Passez-moi l’expression.
Allow me the expression.
