This is driving me nuts. I can Copy from Downloads but can't Paste into a common folder like Videos. Yet I can Drag from ont folder to the other folder. This is Mint 18.3 and it's happened on two different machines with a fresh Mint install, so I don't think it's the system.

System: Host: jim-Aspire-XC-603G Kernel: 4.13.0-38-generic x86_64 (64 bit gcc: 5.4.0)
Desktop: Cinnamon 3.6.7 (Gtk 3.18.9-1ubuntu3.3) dm: lightdm
Distro: Linux Mint 18.3 Sylvia

Are you copying from the default user Download directory to the default user Videos directory? What do you mean by common folder. As a user, you should have read write access to anything in your /home/user directory and not much else. You see to be saying that you cannot copy and paste from your user Download folder to your same user Videos folder but that you can drag and drop from/to those folders. That would by pretty unusual. You might be a little more specific about whether these directories/folders are in the same user /home directory. If they are not that is expected behavior. Have you read the Mint User Guide?

https://linuxmint.com/documentation.php

Yes, downloads and videos are both in my home directory and look to have the same permissions. Although file permissions have a - in the box and folder permissions have an x for read and write. I'm not sure what that means. It just seems odd that paste is greyed out but I can drag. Is this a peculiarity of mint 18.3?

If you open a terminal and run the command: ls -l it should output the permissions as well as the owner/group. Check that and you should see that all files start with a dash ( - ). Not sure what x you are referring to. Are you doing this in your file manager? Directories such as Downloads and Videos in your /home/user directory should have the same owner/group and permissions.

Just to clarify, you right click on a file in Downloads directory, then go to Videos and right click and the Paste option is greyed out?

are you right click copy . ctrl+v in the the directory you're wanting to put them in, or using keyboard short-cuts for both actions 'ctrl+x ctrl+v, or file manager drop down edit->copy->paste. what method are you using? can you copy them over in a terminal?

Directories have to have an x with them to or you will not be able to access them.

Copy and paste and drag-and-drop work fine in my Mint 18.3 installation, permissions on both Downloads and Video using <right-click - Properties> are:
Any help?

Play Bonny!

Can you please, action by action, take us through the process with which you're having problems.

Sometimes with guis if you copy with CTRL-c and then try to paste with right-click > paste, paste is greyed out.

If you are referring to the File Access option as shown in the post above by Soadyheid, what I see on Mint Mate 18.3 is the same. The other options available for File Access are read only, read and write for the user, None, read only and read write for group and others. If you are seeing an x in File Access, I'm not sure how you managed that. Posting the output of ls -l on that directory might be informative.

Yes, that is correct. Paste is there but grayed out. This is on a fresh install of Mint 18.3 and I haven't changed permissions on those folders.

Yes, that is correct. Paste is there but grayed out.

this should be what your directory looks like when you open a terminal and ls -la to look at its permissions

Run the following commands. Post the output here.

That should provide some useful info, replace username in the commands with your actual user name. I don't know what the 'x' would mean for read/write options.
Does your problem work in the reverse, copying/pasting from Videos to Downloads?