If conditional "didn't" vs "hadn't"

Solution 1:

The past simple in if-clauses has three functions:

1. to indicate that something is not true (contrafactual):

   • If I had time, I would help you. (but I don't have time)
   • If I didn't know you were joking, I'd be angry. (but I do know you're joking)

2. to hypothesise about the future (but conveying that you regard the possibility as remote):

   • If I won a lot of money, I'd retire.
   • If I didn't attend the meeting, I'd probably get the sack.

3. to refer to habitual past events:

   • If I was late, he would make me work through lunch.
   • If I didn't eat my greens, I got no dessert.

Your sentence (If I didn't come to the meeting, it wouldn't happen) belongs either in category 2. For example:

• If I didn't come (go is more likely) to the meeting, it wouldn't happen (e.g. I wouldn't get my monthly bonus.) - but I'm definitely planning to attend.

Or in category 3:

• If (whenever) I didn't come to the meeting, it wouldn't happen (I wouldn't get my monthly bonus.)

The sentence If I didn't come to the meeting, it wouldn't have happened is ungrammatical.

Solution 2:

"If I didn't come to the meeting" could reference the future.

Solution 3:

Your first sentence is an example of what you may have heard called the Third Conditional. It is used to express something that didn’t actually happen. Your second sentence, on the other hand, is an example of the Second Conditional, which expresses something that is still possible, but which is unlikely.

It’s easier to see the difference with sentences that are positive rather than negative, as these three examples may show.

First Conditional: 'If you run you will catch the train'. This predicts a likely event: running will certainly allow you to catch the train.

Second Conditional: 'If you ran, you would catch the train.' This suggests that the person addressed is unlikely to run.

Third Conditional: 'If you had run, you would have caught the train.' The person addressed didn’t run.