Sur vs. sûr
The French words sur and sûr are homophones, meaning they sound identical. However, the presence of the circumflex accent (^) on sûr completely changes its meaning and grammatical function.
Sur
A1This is a preposition that most commonly means on or upon. It is invariable, meaning it never changes its spelling. It can also be used to mean about (a topic), over, or out of (in a ratio).
Le livre est sur la table.
(The book is on the table.)
J'ai lu un article sur l'écologie.
(I read an article about ecology.)
Le balcon donne sur la cour.
(The balcony overlooks the courtyard.)
Une personne sur dix a vu ce film.
(One person out of ten has seen this movie.)
Cliquez sur ce lien pour continuer.
(Click on this link to continue.)
Il pleut sur la ville.
(It is raining over the city.)
Sûr
A2This is an adjective meaning sure, certain, confident, or safe. As an adjective, it must agree in gender and number with the noun it describes. The forms are sûr (masculine singular), sûre (feminine singular), sûrs (masculine plural), and sûres (feminine plural).
Es-tu sûr de vouloir partir ?
(Are you sure you want to leave?)
Elle est sûre de sa réponse.
(She is sure of her answer.)
C'est un endroit sûr pour cacher la clé.
(It's a safe place to hide the key.)
Nous sommes sûrs qu'ils arriveront à l'heure.
(We are sure that they will arrive on time.)
Ces informations sont-elles sûres ?
(Is this information reliable (sure/safe)?)
Bien sûr, je peux t'aider.
(Of course, I can help you.)
Summary
In short, the key difference is the accent and the part of speech. Sur (no accent) is an invariable preposition meaning on. Sûr (with a circumflex accent) is an adjective meaning sure or safe and must agree in gender and number with its noun. The circumflex accent is not optional; it is essential for the meaning.