How we regularly talk is in the indicative mood. It is used to state something seen as a fact.
The subjunctive is a mood used to express desires, doubts, emotions and the unknown. This is NOT a tense! More info about the Subjunctive can be found here.
The imperative mood is just commands (: It is used to tell someone what to do. For this reason, there is no yo form in the imperative!
If you don't know which mood to use, just use indicative!
Past Subjunctive (also called imperfect subjunctive)[edit source]
Beber in the Past Subjunctive
bebiera
bebiéramos
bebieras
bebierais
bebiera
bebieran
**Please note: There are two past subjunctive conjugations! The table above depicts the series of endings most widely used today, but below you'll find the other conjugation. The conjugation below is mostly found in older Spanish literature. **