Étang vs. mare
While both étang and mare refer to bodies of standing water, the main difference lies in their size and depth. An étang is a significant body of water, often comparable to a small lake, whereas a mare is a much smaller, shallow pool often found in gardens or fields.
Étang
B1An étang refers to a large pond or a small lake. It is big enough to maintain a permanent ecosystem with fish and is often used for activities like fishing or light boating. Generally, you cannot see the bottom in the middle of an étang.
Nous avons passé la journée à pêcher au bord de l'étang.
(We spent the day fishing by the edge of the pond.)
Cet étang est assez grand pour faire du pédalo en été.
(This small lake is big enough to go paddle boating in the summer.)
Le château est entouré d'un magnifique étang artificiel.
(The castle is surrounded by a magnificent artificial pond.)
En hiver, la surface de l'étang gèle et on peut parfois patiner.
(In winter, the surface of the pond freezes and sometimes you can skate.)
Mare
B2A mare is a small, shallow pool of water. It is typically stagnant and might even dry up during hot summers. A mare is usually too small for fish but is a perfect habitat for frogs, insects, and ducks.
Les enfants observent les têtards dans la mare au fond du jardin.
(The children are watching the tadpoles in the pool at the bottom of the garden.)
Après l'orage, une grande mare de boue s'est formée sur le chemin.
(After the storm, a large mud puddle formed on the path.)
Il y a une petite mare aux canards près de la ferme.
(There is a small duck pond near the farm.)
Cette mare est trop petite pour y mettre des poissons.
(This pool is too small to put fish in it.)
Summary
To choose the correct word, look at the size of the water. If it is substantial, deep, and large enough for a boat, use étang. If it is a small, shallow water hole in a garden or field, typically inhabited by frogs rather than fish, use mare.