can I install an old version of windows 10 inside a newer one,without

Hi, you can only go back to an older build of the same major build - an in-place upgrade repair install- if you wish to keep all progs and data. That requires that you can boot normally.

I.e. if you have, say, 1903 installed, you cannot go back to 1809 or 1803 without clean installing. But you can use any iso of a 1903 build, older or newer - and one of any later major build.

Upgrades cannot be done by booting from an external disk.

every time I have boot problems,I don't want to lose a lot of time trying to fix them.

The prime way to give yourself the ability to go back to a previous good working state if something goes badly wrong- even if your disk fails or becomes unbootable- is ot use disk imaging routinely and regularly as is so, so, so often recommended here.

Then you can restore a previously created image of any disk or partition as necessary. This also acts as a full backup from which you can extract files and folders if you wish.

E.g. Macrium Reflect (free-paid) + large enough external storage for image file sets.

It sounds as if you are not doing this yet.

Boot problems: two basic possible tools:
a. Boot from a Win 10 install disk, via Repair your computer, to Startup repair.
b. Boot from Macrium Reflect boot disk and use the Fix boot tool.

Thank you for your reply. No comment on the above?

There is a procedure that works if your Windows partition is intact - the registry is not damaged.

a. Create a disk image of your existing 'C: '
b. Clean install the same build of Win 10 as your installation.
c. Check that boots.
d. Restore your image of C: in place of the newly created one.

You should not be having frequent failure to boot problems. I've 2 PCs - been running Win 10 for several years. No such failure. Have you checked your disk?

I don't want that it removes apps and settings. I don't want to restore an old image of windows. I'm thinking to create a new tool,that does not exists yet. the iso image of the current windows installation. when windows asks what I want to do,I will say that I want to do a new installation. But since the iso image contains all the settings,apps and programs that I'm using,it will replace everything with the same informations. But for sure,it will fix also the boot files correctly.

Just a comment, or rather a remembrance, the "old" axiom in dual-booting was to install the oldest version first then the newer one.