pretérito imperfecto de subjuntivo (cantara or cantase), some examples:
1. ORACIONES COMPLETIVAS (PASADO)
Andrés me ha dicho / me dice / me esta diciendo (ahora) que vaya con él (ahora) , He is telling me (now) to go with him (right now). PRESENT
Andrés me dijo / me había dicho (ayer) que fuera / fuese con él (ayer). He told me (yesterday) to go with him (yesterday). PAST
2. CONDICIONAL
Si Andrés estuviera / estuviese (verbo estar) aquí, su tienda estaría abierta. If Andrés was here his shop would be open
3. USO LITERARIO
"... y los exploradores tuvieron que regresar al refugio que abandonaran dos días antes." This form can be replaced by "habían abandonado" or "abandonaron" (formas del pasado de indicativo) It only must be used in written Spanish (and only the form ending by -ran, never -sen) and it's too ...mmm... litterary, snob? Anyway, it's quite rare (in Spain, at least)
*** notice that sometimes the form that has been explained and future are alike.
"abandonaran" pretérito imperfecto de subjuntivo vs "abandonarán" futuro imperfecto de indicativo
*** there is also futuro imperfecto de subjuntivo (cantare), which is used no longer in current Spanish. It's different from cantaré (future)
I hope this helps you (sorry for my translations to English)
AIXA, Spain