Hardware space.... Clearing memory

Hi All & thank you for reading this post.

I am new to the mac and therefore really appreciate any advice i can gather from here.

Right, so i have a mac desktop that we use for editing videos at work (basic videos), the desktop has a hardware capacity of just under 1000 GB and i have 21 GB left. There are several strange goings on that i have never experienced before and shouldn't be happening, i believe they are a consequence of the lack of space left on the desk top....

I recently bought an external hard drive and have moved 'what i thought was everything' onto the external hard drive... I thought that this would completely clear the desk top space and give me a 'clean mac', again! But no!
Why is this the case? I have cleared my junk box/trash can... And don't many applications...

What is the issue? How can i get the 1000 GB space back?

Many thanks!!
Ed

Are you absolutely sure you emptied the trash?

- Nick

are you sure you moved them, not just copied?

When you move items off your computer, it's sends a copy to the external, not the original file. After everything is on the external drive, you need to delete what you don't want on your Mac in the trash can.