Your ET1 is the form you complete to start your claim — also known as the claim form.
It’s a vitally important document and it’s important you make sure you keep a copy of it. If you’re submitting online, you can do this by saving it to PDF when submit it electronically. If you post it, you’ll need to take a photocopy.
This all sounds very obvious, but I’m encountered a surprising number of cases where people have sent off their ET1, without holding onto a copy. I suspect the problem is that the wider world of the internet has taught us all that we’re bound to receive a confirmation email with the form attached. Unfortunately you don’t.
If you don’t keep a copy, contact the tribunal and ask for one.
On no account try to reproduce your original ET1 by filling it in again from memory. This will a) not work (you’ll never remember it exactly) and b) risks looking as if you’re trying to pull a fast one somehow.