Mixed conditionals

                                    If + Past Perfect (had+past participle), Would (n't) + infinitive 

Use: This type of mixed conditional refers to an unreal past condition and its probable result in the present. 

Example:

If I had written a list, then I would buy everything I needed.

If + Past Simple, Would (not) have + past participle+ infinitive 

Use: To talk about the past result of an unreal or hypothetical present situation.

Example:

If I were definitely more organised, I would have made a chart with all the tasks I must complete. 

Here's a video that might be helpful:

And, of course, after theory, let's have a look at some practice: 

At the end of this page, you have a To Do part. Four simple exercises to help you check your knowledge: https://www.bbc.co.uk/learningenglish/course/towards-advanced/unit-7/session-1