at is a preposition, most commonly translated in French by à.
It locates a point in time or space.
Unlike to it is never used with motion verbs like come or go (or welcome).
In general, it is more precise than "in" (see the exception in #2 below)
- at 8 o'clock, (be) at the train station
- at night (NB: this is unusual: in the morning / afternoon / evening)
It is also used in many idiomatic expressions:
- at best / at worst: au mieux, au pire
- at ease
- at first: de prime abord
- at risk
- at will