Concentrer vs. se concentrer
The key difference between concentrer and se concentrer lies in their grammatical structure. Concentrer is a transitive verb that requires a direct object (you concentrate *something*), while se concentrer is a reflexive verb where the subject acts upon itself (you concentrate *yourself*).
Concentrer
B1This is a transitive verb meaning to gather, to focus, or to bring something together in a single place or on a single point. It always requires a direct object; the action is done *to* something or someone else.
La loupe peut concentrer les rayons du soleil.
(The magnifying glass can concentrate the sun's rays.)
Il a dû concentrer tous ses efforts pour réussir l'examen.
(He had to concentrate all his efforts to pass the exam.)
Le coach a décidé de concentrer la défense sur le côté gauche.
(The coach decided to concentrate the defense on the left side.)
L'entreprise va concentrer sa production dans une seule usine.
(The company is going to concentrate its production in a single factory.)
Se concentrer
A2This is a reflexive verb. Its most common meaning is for a person to focus their mind and attention. It can also be used more broadly to mean to be gathered or to be centered in a location.
Je dois me concentrer sur mes devoirs maintenant.
(I have to concentrate on my homework now.)
Avec tout ce bruit, il est difficile de se concentrer.
(With all this noise, it is difficult to concentrate.)
La richesse se concentre dans les quartiers nord de la ville.
(The wealth is concentrated in the northern districts of the city.)
Concentre-toi, tu vas bientôt trouver la solution !
(Concentrate, you will find the solution soon!)
Summary
In short, use concentrer when you are focusing an external thing (e.g., efforts, troops, rays of light). Use se concentrer when you are talking about focusing your own mind or attention. If you want to say I need to concentrate, the correct French phrase is Je dois me concentrer.