Idée vs. notion
In French, both idée and notion can translate to "idea" or relate to having a "clue" or understanding, but they differ significantly in scope and context. While idée is the everyday word for a spontaneous thought, an opinion, or a general awareness of a situation, notion refers to a more abstract concept, a theoretical principle, or foundational knowledge in a specific subject.
Idée
The word idée is the most common and versatile term for a thought, a mental image, or a suggestion. When referring to knowledge or having a clue, idée represents a general, practical awareness or understanding of a situation, and is frequently used in negative sentences to express having absolutely no clue.
Je n'ai aucune idée de la façon dont ce problème peut être résolu.
(I have no clue how this problem can be solved.)
As-tu une idée du nombre d'invités qui viendront ce soir ?
(Do you have an idea of the number of guests coming tonight?)
C'est une excellente idée de commencer par la fin du livre.
(It is an excellent idea to start with the end of the book.)
Donne-moi au moins une petite idée de ce que tu veux pour ton anniversaire.
(Give me at least a little clue of what you want for your birthday.)
Il se fait une fausse idée de ce qu'est le travail indépendant.
(He has a false understanding of what freelance work is.)
Elle est partie sans que j'aie la moindre idée de sa destination.
(She left without me having the slightest clue of her destination.)
Notion
The word notion is more academic and formal. It refers to a conceptual idea, a theoretical principle, or the fundamental basics of a topic. When used to mean "clue" or "knowledge," it usually implies having a foundational grasp or basic skills in a specific field, and is often used in the plural form.
La notion de démocratie a beaucoup évolué au fil des siècles.
(The concept of democracy has evolved a lot over the centuries.)
Pourriez-vous m'expliquer cette notion mathématique plus simplement ?
(Could you explain this mathematical concept to me more simply?)
Il possède de très bonnes notions en programmation informatique.
(He has a very good basic knowledge of computer programming.)
Je n'ai pas la moindre notion de physique quantique.
(I do not have the slightest clue about quantum physics.)
Le professeur a introduit de nouvelles notions pendant le cours de ce matin.
(The teacher introduced new theoretical ideas during this morning's class.)
Perdre la notion du temps est fréquent quand on lit un bon roman.
(Losing the concept of time is common when reading a good novel.)
Summary
To summarize, use idée for everyday thoughts, suggestions, or general awareness, especially when saying you have "no clue" about an everyday situation. Opt for notion when discussing abstract theoretical concepts or when referring to basic foundational knowledge and skills in a specific subject.