I've recently moved my Chinese Elm outside the window, luckily out there he gets a lot of sun, inside the house where I kept him he only received 2-3 hours of direct sunlight.

He's behaving weirdly, behavior that started before I moved him outside, a lot of new shoots are dying before they can grow to maturity but he still keeps on growing more and more, he's watered correctly, I check the soil everyday and when I feel it's dry I give him some water.

Now about doormancy, I live in Eastern Europe, northern Romania, summer here is extreme as well as winter, during the day you get temperatures above 30C and at night 15-18C during summer, winter on the other hand goes as low as -20,-25C which from what I believe is a little too cold for my Chinese Elm.

My questions are:
1. How do I make sure he's not gonna die during winter outside, at some point I should bring him inside ?
2. Why are new shoots dying off before they reach maturity ?

The behaviour you show is a combination of light and roots. IN your situation, you have plenty of lights. So roots it is. It looks like you have the plant in the muck the dealers call soil. It is very dense, dries out to a hard ball and hardly allows for any air circulation. If you have a larger pot, and good quality potting soil, consider taking this out of the current pot. Scrats the first half cm to cm of the current rootbal so it goes fluffy. Then repot it in the larger container, using good quality potting mix. In spring, clean out the rootball you have now, and replace with something more open, well draining. E.g.: www.growingbonsai.net/olive-pre-bonsai-repotting/

For winter.. Yeah, I would say that is too cold. What you can do, is leave it outside till the first frost. Then it will drop all leaves. Then you move it to an unheated garage, where temps stay below 6, 7C. That will keep the tree dormant. Try to protect it for temps below -5, -10C. An odd night of that sort of cold is no issue. Prolonged it migh be.

Else you can keep it indoors for winter, in plenty of sun. But that is a route to slow detoriation, in most cases.

Sadly I've no garage to place it after the first frost days, it's gonna be either outside where it's chilly or inside where it's 24+

Gonna have to figure out where to put him, after reading this guide:
www.evergreengardenworks.com/dormancy.htm

Maybe put him in the fridge for a few weeks so it stays doormant ?

As for the repotting, i'll have to wait until spring as you said to do that, right ? Right now it's a bit too late

And thanks for the quick reply @leatherback!

Or you take a gamble, and put a big bucket of mulch around the tree once dormant and leave it outside. Accourding to this site, they should be able to withstand a lot colder than we assume normally: extension.illinois.edu/treeselector/deta...lant.cfm?PlantID=280

That sounds awesome but sadly I live in a small studio and the only place outside I have is a small edge in front of my kitchen window where the Elm is now sitting, the edge is about 20cm wide so not a lot of room for doing that.

I still think putting him in the fridge wrapped with a thin plastic bag would do the trick after the first frost, water every 2 weeks ?

What about wrapping the pot in burlap material? Or even layered with polystyrene and then wrapped in burlap. No guarantee, but it might give the tree a chance.

That doesnt sound so bad actually, thanks for the tip, how long should I keep like that during the winter ? 20-30 days ?

Depending on your carpentry skills you could build a frame around the shelf, cover the frame with plastic, and let it take some heat from the window through the glass. It will get plenty cold, but the plastic will keep the wind off and keep the temperature from being too cold. Put something insulating on the bottom, too. You can put a thermometer by the pot to see what happens.