Hello Carol, I see you just start with Spanish, so I'll make the answer very simple. If I'm wrong, and you are (at least), an B2 student, then we can go more in details.
The forms -se and the form -ra are completely interchangeable -as long as the meaning is completely subjunctive-.
Because the form -ra can sometimes replace the conditional or the pluperfect of the indicative, for example: No cumplió la promesa que me -hicieRA- antes de casarse = No cumplió la promesa que me había hecho antes de casarse.
According to the RAE, -es is mostly used for common people, and -ra is more common in literature. I personally disagree, but you can choose which one you prefer as long as you remain in the subjunctive ;)
Best regards,
Alejo.
REF.: I'm a Certified Spanish Teacher.