There are several confusing pairs and difficult expressions related to French numbers.
Floor System Differences
The numbering system of the floors in a building may be difficult for American English speakers.
American English British English
Rez-de-chaussée First floor Ground floor
Premier étage Second floor First floor
Deuxième étage Third floor Second floor
The meaning of premier changes depending on the preposition that precedes it.
Au premier At/On the first
Au premier étage On the second floor On the first floor
En premier The first in a sequence
En premier lieu In the first place, Firstly
De premier/première (prix, qualité) The top (prize) or best (quality)
Two Words for the Same Number in French
There are two French words for the English ordinal number "second" (2nd): second and deuxième. They are essentially interchangeable, but there is a general, not always followed guideline:
Second is often used when something is the second in a series of only two things.
Deuxième tends to be used when there's a third, fourth, etc.
Following this guideline allows you to add some interesting nuances to your words.
La seconde guerre mondiale World War II (the second and last)
La deuxième guerre mondiale World War II (the second of how many?)
There are two words for "third" and "fourth" - one for the ordinal number and another for the fraction
troisième third (in a series)
un tiers one third
quatrième fourth (in a series)
un quart one fourth