In classic MacOS, and until fairly recently I think, you could have it that when you double click a folder it opens in a new window so you can drag things between folders easily. Since upgrading to Mojave I now don’t seem to be able to enable that as the default and instead have to hold the option key down. Does anyone know if I can restore the function to always open in new window in the finder?
I am a bit confused here. “Classic MacOS” was the early Mac operating system(s) up to and including MacOS 9 so you likely were not jumping from that to Mojave. 
I honestly cannot recall the last time when double-clicking a folder would always open a new window in the manner you describe. In my memory, typical behavior, assuming the Finder is not set to open new folders in Tabs, is if a folder is open and contains more folders then double-clicking one of those “internal” folders open in that same window.
To open a new window to allow for the side-by-side option you mention the Command Key needs to be held down while double-clicking the folder. That will then open that folder in a new window while preserving the visibility of the original folder window.
Your mention of holding down the Option Key is not designed to achieve what you seem to want. Holding down the Option Key while double-clicking a folder within a folder should then close the original folder and subsequently open the folder just double-clicked in a new window.
Is it possible you have a third-party application installed that has allowed you to manipulate folders differently than the default for current versions of MacOS?
If you are dragging things frequently between folders have you considered using Column View?
Up until Mojave you could open a folder in a new window by simply holding cmd when you double-click it. If you have Finder prefs set to open in tabs instead of folders, that would open the folder in a new tab.
I have not touched Catalina yet so I cannot check if that behavior has changed there.
It you turn off the toolbar for a Finder window, double clicking on a folder will open the folder in a new window.
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Thats great, that’s what I was looking for as I was sure it still existed in OS X
Yes, thanks for this reminder/tip. I had completely forgotten about this, probably because I reply on the sidebar and toolbar for too much navigation to eliminate both of them. Once again, a nice set of options from Apple that satisfy many different user’s workflows.
If you hold down the Option key when you single-click on a folder, at the top of the context menu you will see an option to open in a new folder. There does not appear to be a keyboard shortcut for the operation, however, as this option does not exist in a menu-bar menu.
However you can add one using System Preferences > Keyboard > Shortcuts. Add a new App Shortcut for the Finder menu “Open in New Window” (exactly, including case, but without the quotes). Interestingly the Finder seems to recognize you’ve done that as the first item in the context menu becomes Open in New Tab when there’s a shortcut for Open in New Window.
I did some more exploring and discovered that, at least in Mojave, command-control-O (oh), does an Open in New Window out of the box, so no need to add a custom shortcut.
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Thanks for all the good tips, everyone. It seems that we all live in the Finder more than we probably realize.
My long-standing challenge is how to open a new tab for the folder I’m currently in. If I choose “New Tab” from the File menu, I get a new tab of my default folder (the one I set in Finder Preferences > General).
I thought I had saw this a few years ago and was able to do it, but now I can’t figure out how I did it, or maybe I just can’t find it because it’s gone.
Any suggestions. Thanks very much.
This is not a real solution to your problem, but it’s a workaround.
Right-click on a folder within the folder you are and select Open in New Tab. That will launch a new tab one folder below where you actually want to be. Then just hit cmd-up which will move you to the parent folder, i.e. where you wanted to go in the first place.
At least, as of Mojave: Show the Path Bar, View > Show Path Bar. Right click on any of the folders shown in the path and choose Open in New Tab.
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In Big Sur (and maybe older systems), the 'Open in New Tab 'option appears if the Finder Preference for 'Open folders in tabs instead of new windows ’ is checked. If you haven’t checked that preference, you can still have it appear by holding down the Option key when right-clicking.
Good advice. 
I can confirm that works in Big Sur too.
Thanks, everyone, for the quick replies and advice. Those are all good tricks, albeit (macOS’s fault, not yours!) a bit kludgy. It’s too bad that I can’t just use a default key command, or assign one, to “duplicate” the tab for the folder I am looking at. I know that I had figured it out once, and had a “Open New Tab” (for this folder) command working a few macOSes ago. I’ll see if I can re-create that… I’ll let you all know!