The correct sentence is:
✔ “I wouldn’t believe it if I hadn’t seen it myself.”
Why?
This is a third conditional sentence, which is used to talk about hypothetical situations in the past.
Structure of the third conditional:
If + past perfect → would have + past participle
However, in this case, we use “wouldn’t believe” instead of “would have believed,” making it a mixed conditional.
• “Hadn’t seen” (past perfect) refers to something that did happen in the past (you did see it).
• “Wouldn’t believe” (conditional) refers to the present (you believe it now because you saw it).
Why are the other sentences incorrect?
❌ “I wouldn’t believe it if I didn’t see it myself.”
• This is a second conditional structure (if + past simple → would + base verb), which is used for hypothetical situations in the present or future, not the past.
• The correct version for present situations would be:
• “I wouldn’t believe it if I didn’t see it right now myself.”
❌ “I wouldn’t believe it if I haven’t seen it myself.”
• This mixes a conditional structure with the present perfect, which is grammatically incorrect.
• “If I haven’t seen it” suggests a real present or past action, but “wouldn’t believe” makes it hypothetical, creating a mismatch.