“I think you forgot to attach the PDF.”
Someone sends you an e-mail message at work. In the message, she writes something about a PDF document that she included with the message, but you don't see any PDF. This is what you write in response.
I think you forgot to attach the PDF.
Want Video and Sound? Follow us on YouTube
forget to (do something)
When you "forget to do" something, it means that you wanted to do something, but you didn't do it because you forgot:
Oh, I forgot to lock the door. Hold on just a second.
Oh shoot. Remy called me yesterday and I forgot to call him back.
Notice that there is a different phrase for when you do something, but then later forget that you did it. That phrase is "I forgot that (clause)":
I forgot that she was a year younger than us. I thought we were all the same age.
I forgot that I had actually met him once before at Crystal's party. So I introduced myself to him again.
attach (a computer file)
The word "attach" means to stick or connect two things to each other. When you're writing e-mail and want to send someone a file, you "attach" the file to an e-mail message. The file that you attach is called "an attachment".