Real vs Unreal Situations

First and second conditionals differ in exactly one way: how likely it is. "If it rains tomorrow, I'll take an umbrella" describes a real possibility you actually expect. "If I won the lottery, I'd quit my job" is purely imaginary and hypothetical.

The verb form signals which world you're in. Present tense after "if" means you genuinely think it might happen. Past tense after "if" means you know it probably won't. Your listener's brain reads this signal instantly and adjusts its expectations.