Reflexive Verbs

Question

If s'inscrire (to register oneself)  uses être as it's a reflexive verb, then why do we say "Le mois dernier, nous avons inscrit Thomas au judo" and not "Le mois dernier, nous sommes inscrits Thomas au judo" ?

Maarten Kamp · Accepted Answer

Mutaal, 
If you register yourself, that is reflexive, as the object and the subject of the verb are the same (person) -  you. 
Only a limited number of verbs are always pronominal.
All pronominal verbs, including all reflexive verbs, conjugate with être as auxiliary. When you register someone else, they are the object of the verb, you are the subject. 
The overwhelming majority of non-reflexive verbs, that is the overwhelming majority of all verbs,  do not conjugate with être - they conjugate with avoir as the auxiliary. 
 https://progress.lawlessfrench.com/revision/glossary/verb-types/reflexive-verbs
 https://progress.lawlessfrench.com/revision/glossary/verb-conjugation-group/verbs-taking-etre-as-auxiliary
 https://www.lawlessfrench.com/grammar/reflexive-verbs/
 https://www.lawlessfrench.com/grammar/pronominal-verbs/
 https://www.lawlessfrench.com/grammar/essentially-pronominal-verbs/