They tightened: they clung in their arms, here “se” is a direct object, since the verb tightening is itself, is for themselves (they have themselves or the other), so there is an agreement: tight! They shook hands, where is the direct object? it is “the hand” (they shook their hands), the direct object is according to the verb not to have a correspondence. In this case, “se” is an indirect object because we can say: they shook hands themselves. So far, the party of pronoun verbs follows a different rule of agreement than the auxiliary “tre,” even though the pronominal form is formed with “Tre.” Overall and before the details, the agreement must be made as if it is the “having” of the aid. In French, all pronoun verbs are used with the help -Tre: Note that certain verbs that are not normally pronouns can be used with a reflexive pronoun to avoid the passive voice. This construction is called passive reflexive. There are some kinds of French pronoun verbs. But in general, we can say that the action and therefore the construction of the verb pronominal is reflexive, recidivist or idiomatic. Note that many reflexive verbs also have non-reflexive use; i.e. they can describe someone who performs the action of the verb to someone or something else: once again, the reflexive pronoun must always correspond to the subject, even if the pronoun verbs are used as present participatorys: the pronoun verbs have a reflexive pronoun that relates to the subject, and this reflexive pronoun represents either the direct object or the indirect object of the verb. Here is the tricky part: the agreement is only necessary if the reflexive pronoun is a direct object; if it is indirect, there is no agreement. So to know if current participants need to agree, you need to determine the function of the reflexive prognosis. That`s how it works.
-Non-reflexive verbs: here `s` does not refer to the subject: She hurt her hand. (She hurts his hand). Other examples: They broke their arms.