solid
The French word 'solide' is used to describe something that is firm, stable, and strong. It can be used to describe both physical objects and abstract concepts. For example, you might say 'un solide argument' to describe a strong argument, or 'une table solide' to describe a sturdy table.
This is a solid matter.
In this example, 'solide' is used to describe physical properties of objects. When a matter is called 'solide' in French, it means that it's hard and firm to touch, similar to the English equivalent 'solid'.
He is solid as a rock.
This is a simile in which 'solide' is used metaphorically to describe someone's strength or resilience. 'Il est solide comme un roc' directly translated as 'He is solid as a rock' indicates this person's unwavering determination or physical strength.
You must be solid in mathematics.
In this context, 'solide' conveys the need for strong skills or proficiency. When someone is 'solide en mathématiques', it implies that they have a good grasp, understanding, and ability in the subject of mathematics.