Reflexive verbs are verbs where the subject and the object are the same person or thing. The object is represented by a reflexive pronoun = representing the same person / thing as the subject. The subject can either be a normal noun or a personal pronoun.
More information about reflexive pronouns can be found here: Reflexive Pronouns.
A true reflexive verb is always reflexive! You can't leave out the reflexive pronoun or replace it with another noun or pronoun.
It is impossible to use "konzentrieren" without the reflexive pronoun!
In the dictionary these words are marked in the following way: "s. konzentrieren," "sich konzentrieren," or "konzentrieren (ref.)"
Be careful, because verbs that are reflexive in German might not be reflexive in your native language. When you learn a new verb, you also have to learn whether or not it is reflexive.
Partial reflexive verbs can be used in both a reflexive or non-reflexive way. It depends on the situation.
The subject can either act upon itself (reflexive pronoun needed!) or someone / something else (no reflexive pronoun!):
The general rule:
Exceptions: When the verb requires a certain case.
In the main clause:
Only the verb goes to the end. The reflexive pronoun stays in position 3.
With modal verbs and helping verbs.
Only the verb goes to the end. The reflexive pronoun stays in position 3 (after the modal verb or helping verb).