Unreal present and past: the imperfect subjunctive earns its keep
Unreal present/future conditions: si + imperfecto de subjuntivo, condicional. You met the chunks at B1 — now own the full pattern.
Si tuviera más tiempo, aprendería a tocar el piano. (If I had more time, I would learn the piano.)
Si fueras más amable, tendrías más amigos. (If you were kinder, you'd have more friends.)
Never the conditional in the si-clause: si tendría is the error that screams "English speaker".
Take the ellos form of the indefinido, drop -ron, add -ra endings. All the indefinido's irregularity comes along free.
hablaron → hablara, hablaras, hablara, habláramos, hablarais, hablaran
tuvieron → tuviera · fueron → fuera · hicieron → hiciera · pudieron → pudiera
There is also a -se form (hablase, tuviese) — same meaning. Recognise both, produce the -ra form.
Unreal PAST: si + hubiera + participio, habría + participio. The grammar of regret and relief.
Si hubiera sabido, no habría venido. (If I had known, I wouldn't have come.)
Si hubieras estudiado, habrías aprobado. (If you had studied, you would have passed.)
Spanish also accepts hubiera in BOTH halves: si hubiera sabido, no hubiera venido — common in speech.
Como si (as if) always takes the imperfect (or pluperfect) subjunctive — the comparison is unreal by definition.
Habla como si lo supiera todo. (He talks as if he knew everything.)
Me miró como si no me hubiera visto nunca. (She looked at me as if she had never seen me.)
como si + supiera (imperfect subj.) for present unreality, + hubiera visto (pluperfect subj.) for past unreality.
Two elegant alternatives for advanced writing and speech.
De haberlo sabido, habría venido antes. (Had I known, I'd have come earlier — de + compound infinitive.)
Yo que tú, aceptaría la oferta. (If I were you, I'd accept the offer.)
De haberlo sabido is formal/written; yo que tú is everyday advice. Both replace a full si-clause.
Traps for English speakers
These are the errors English speakers make most often.