Zero and First conditional shows something will happen in certain degree ,but I don't get about Second Conditional and Third Conditional.
Both of them are impossible and unreal, what is the difference?
1 - If + Past tense, subjunctive (irreality)
2 - If + Past Perfect tense, subjunctive (irreality referring to past time)
In 1 you speak of now, in 2 you speak of something in the past. The asterisk* marks "had" after "if" as subjunctive form.
I would forget those terms conditional 2 and conditional 3, they will never give you an understanding of the grammar system.
If we use the following notation
Pt for Past, Pt* for Past subjunctive
PP for Past perfect, PP* For Past perfect, subjunctive
C1 for Conditional (would + bare infinitive)
C2 for Conditional perfect (would + bare infinitive perfect)
you have the following structure
In 1: If + Pt*, C1
In 2: If + PP*, C2