Hello,
I had some files in a folder and I decided to go tagging only. I tagged the files in this folder with the tags I needed, and thinking that in DT tags=folders I deleted the old folder (I thought it would be the same as just deleting a tag). As long as I didn’t empty the trash everything continued to work ok, but when I emptyed the trash my files disappeared from the tag-groups too. What did I do wrong?
Shouldn’t the tags work as groups, so if I have a file in a folder “foo” that I also tag as “foo2”, if I delete the folder “foo” with its files I would still find the file under “foo2” ?
No.
As you discovered, if you tag a file that’s within a group, then delete the group, the file will be sent to the trash. When the trash is emptied, the file will no longer be in the database.
It is possible to have all of the documents in a database outside of any group organization, stored at the top level of the database. There’s a command to ungroup documents that are located within an existing group. Then one could assign tags to these "no location’ documents.
Personaly, I find value in both group locations and in tags, although I confess that I don’t spend much time diligently organizing content into tightly subdivided groups, and I never bother to tag content as it’s added. But that’s just me. 
Why bother to remove the existing group structure? All you need do to work with Tags without displaying ‘group tags’ is to choose the option in File > Database Properties to exclude groups from tagging.
I would advise against ‘physically’ locating documents inside the Tags groups, as I think that can create logical messiness, especially in hierarchical Tag groups. Tag groups should contain only replicants of items that are located elsewhere (including ungrouped items), and those replicants should be created by assigning tags to the items in the database.
I had just a couple of files in a folder when all the others files were only tagged, so it was just for it to be consistent with the rest of the database. But I have the same problem when I import a file via the inbox, tag it and then delete it… I see I can move a file to be located under a tag but not by using drag and drop which was the method I used. So I guess I need to locate all my files in a folder somewhere…
I must say I would have preferred a system where the file would be “located” in multiple places (every tag), working in the same way as hard links works in unix. There a file would not be deleted before the last hard link would be deleted. That seems to me also to be a safer way of functioning.