Geçmiş zamanlı dilek-şart kipi
To say 'if I had done X, I would have Y' — a counterfactual about the past — Turkish stacks two pieces. The 'if' clause uses the conditional -se / -sa plus the past -ydi / -ydı / -ydu / -ydü: gelseydim = 'if I had come'. The 'then' clause typically uses the aorist past (-er + -di): görürdüm = 'I would have seen'. Gelseydim, görürdüm. = 'If I had come, I would have seen.'
The if-clause: -seydi / -saydı
Stem + -se / -sa (harmony) + -ydi (the past copula 'idi' fused) + person ending. gel-se-ydi-m = gelseydim ('if I had come'). The full set: gelseydim, gelseydin, gelseydi, gelseydik, gelseydiniz, gelseydiler. Front-vowel verbs use -seydi; back-vowel verbs use -saydı: yapsaydım ('if I had done'), olsaydı ('if it had been').
The then-clause: aorist + -di
Stem + aorist (-r / -ır / -ir / -ur / -ür / -er / -ar) + past -di + person ending. gör-ür-dü-m = görürdüm ('I would have seen / I used to see'). This same form covers two meanings — the counterfactual ('would have') and the habitual past ('used to'). Context decides; pairing it with a -seydi clause forces the counterfactual reading.
Negative versions
Negate the same way you negate any verb. gelmeseydim = 'if I had not come'. görmezdim = 'I would not have seen' (note the aorist negative is -mez / -maz, not -mer). Putting both together: Gelmeseydim, onu görmezdim. = 'If I hadn't come, I wouldn't have seen him.'
Compound verbal tenses — the bigger pattern
-seydi is just one slice of a productive pattern: any verbal nominalizer + past copula -ydi = a compound past. gelirdim (would-come / used to come) = aorist + past. geliyordum (was coming) = present continuous + past. gelmiştim (had come) = perfect + past. The pattern stacks the same way for the conditional: gelseydim = conditional + past. Once you see this lego logic, half the 'irregular' Turkish tenses click.
Real vs unreal conditions
-(y)se alone (without past) gives a 'real' condition: Gelirsen, görüşürüz. = 'If you come, we'll meet.' -seydi turns it counterfactual: Gelseydin, görüşürdük. = 'If you had come, we would have met.' Same suffix family, two very different feelings. Good rule of thumb: -se = possible, -seydi = impossible (or didn't happen).
