I don't understand the Subjunctive tense

Try to think of the subjunctive as a feeling/mood/sentiment instead. A tense is an action whereas the subjunctive is a feeling... it is also used to express doubt/uncertainty whereas other tenses are facts. 

eg. 
(Present)

Pedro le gusta la musica
Pedro likes the music (FACT)

(Subjunctive)
I hope that Pedro like the music 
Espero que a Pedro le guste la musica (FEELING)