Conditional Sentence Type 1
→ It is possible and also very likely that the condition will be fulfilled.
Form: if + Simple Present, will-Future
Example: If I find her address, I’ll send her an invitation.
Conditional Sentence Type 2
→ It is possible but very unlikely, that the condition will be fulfilled.
Form: if + Simple Past, Conditional I (= would + Infinitive)
Example: If I found her address, I would send her an invitation.
Conditional Sentence Type 3
→ It is impossible that the condition will be fulfilled because it refers to the past.
Form: if + Past Perfect, Conditional II (= would + have + Past Participle)
Example: If I had found her address, I would have sent her an invitation.