Todorov's Stages
Todorov had a theory that each films plot goes through 5 stages.
1) Equilibrium - where everything is normal and all is well in the story.
2) Disruption - something in the story goes wrong and creates a problem
3) Realisation - the characters realise something has gone wrong/ there is a problem
4) Repair - they tempt to repair the damage done and solve the issue
5) Restoration/New Equilibrium - this may be different to the original and may not always be a 'happy ending' but it will be the new normal
Here are Todorov's theory for 3 Romantic Comedy's trailer vs the actual film as that is my chosen genre.
As you can see it is very uncommon for the trailer to give away the whole story therefor most only include 3-4 stages and never usually the final stage, this is so that it doesn't ruin the film for the audience. however some are different and show some of the ending but not the whole thing, this can include leaving the audience on a cliffhanger to encourage them to go and watch the film.
In terms of my own trailer it has let me understand that i need to have a clear concept of my entire plot before making the trailer so that when I come to making the trailer I can reduce it down into the synopsis for my trailer and only include the important bits.