“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.
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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.
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".