Second vs. seconde
The French words second and seconde both translate to second. The primary difference between them lies in grammatical gender, but seconde also has specific noun uses that do not apply to second.
Second
A2Second is the masculine adjective for second. It is used to describe a masculine noun that is the second item in a series, particularly when there are only two items in total. It can also be used as a noun to mean an assistant or helper, especially in a duel or a boxing match.
Il habite au second étage de l'immeuble.
(He lives on the second floor of the building.)
C'est le second volume d'une encyclopédie en deux parties.
(It is the second volume of a two-part encyclopedia.)
Le Premier ministre a rencontré son homologue pour un second entretien.
(The Prime Minister met his counterpart for a second interview.)
Le boxeur a écouté les conseils de son second entre les rounds.
(The boxer listened to his second's advice between rounds.)
Le Second Empire en France a été dirigé par Napoléon III.
(The Second Empire in France was led by Napoleon III.)
Seconde
A1Seconde is the feminine adjective for second, used to describe a feminine noun. Crucially, seconde is also a feminine noun with distinct meanings: a unit of time (a second), a unit of angular measurement, and the first year of high school in France (equivalent to the 10th grade in the US).
Tournez dans la seconde rue à gauche.
(Turn into the second street on the left.)
C'est la seconde fois que je te le dis.
(This is the second time I'm telling you.)
La Seconde Guerre mondiale a été un conflit global.
(The Second World War was a global conflict.)
Attendez juste une seconde, j'arrive tout de suite.
(Wait just a second, I'll be right there.)
Ma fille entre en seconde l'année prochaine.
(My daughter is starting tenth grade (seconde) next year.)
Un degré est divisé en soixante minutes, et une minute en soixante secondes.
(A degree is divided into sixty minutes, and a minute into sixty seconds.)
Summary
In essence, the choice between second and seconde as an adjective depends on the gender of the noun it modifies: second for masculine nouns (e.g., le second train) and seconde for feminine nouns (e.g., la seconde voiture). However, the most significant difference is that only seconde is used as a noun to denote a unit of time (une seconde) or the tenth grade in the French school system (la classe de seconde). In these specific contexts, second is never used.