Pomme de terre vs. patate
In French, both pomme de terre and patate translate to potato. However, they are not always interchangeable, as they differ in their level of formality and also have distinct figurative meanings.
Pomme de terre
A1This is the standard, neutral, and most common word for potato. Its literal translation is apple of the earth. It is the appropriate term to use in all contexts, from formal writing and restaurant menus to everyday conversation.
Pour le gratin, il faut couper les pommes de terre en fines tranches.
(For the gratin, you have to cut the potatoes into thin slices.)
Je vais acheter un filet de pommes de terre au supermarché.
(I'm going to buy a bag of potatoes at the supermarket.)
Le plat du jour est un steak-frites avec une purée de pommes de terre maison.
(The dish of the day is steak and fries with homemade mashed potatoes.)
La culture de la pomme de terre est très répandue dans cette région.
(Potato cultivation is widespread in this region.)
Patate
B1This is the informal, colloquial term for potato. It is widely used in casual, spoken French among friends and family. Beyond the vegetable, patate has several common figurative meanings, which pomme de terre does not. It can refer to a head, an idiot, or be used in expressions related to energy or feelings. Note that patate douce is the standard term for sweet potato.
Ce soir, on mange des grosses patates farcies.
(Tonight, we're eating big stuffed potatoes.)
Quel patate, il est tombé dans le panneau !
(What an idiot, he fell right into the trap!)
J'ai bien dormi cette nuit, j'ai la patate ce matin !
(I slept well last night, I'm full of energy this morning!)
Il en a gros sur la patate depuis qu'elle l'a quitté.
(He's been very downhearted since she left him.)
Attention à ne pas te prendre un coup sur la patate.
(Be careful not to get a blow to the head.)
On va faire des frites avec le reste du sac de patates.
(We're going to make fries with the rest of the bag of potatoes.)
Summary
In essence, use pomme de terre as the default, standard word for potato in any situation. Use patate in informal, casual conversations, much like using spud in English. Be aware that patate also carries various figurative meanings, such as idiot or head, and is used in popular expressions related to energy or sadness, making it a more versatile but context-dependent word.