How can I save my big tree!?

I can't tell what type of plant it is, but a lot of outdoor plants that are brought inside go into a dormant state for the winter and look dead. However, come spring you can start watering it and fertilizing it and it will fully come back. I'd suggest that you look it up using the actual name of the plant. Search something like - Why does __Plant Name___ have brown leaves when brought inside for winter

I hope that helps. Wishing you the best.

Make sure it gets light, keep it watered enough so the roots do not dry out and consider a humidifier for the room that it is in.

I agree with Naomie suggested. It will go kind of dormant until it adjusts to the condition change of bringing it inside. I would suggest getting a moisture meter, also, so that you know you are not over or under watering it due to the now dry heat it is getting, it may need a little more frequent watering depending on the soil conditions the plant requires, i.e.. letting the soil dry out or likes moist conditions all the time. I would give it time before introducing fertilizer to let it adjust fully. Look up the plant and it should let you know how to care for it after it is brought in for the winter. I use gardeningknowhow.com and the spruce.com for most of my gardening questions for specific plants. The are both good.

Hello. I would mist it daily with a spray bottle, and fertilize. Also make sure you don't have it near a heat register.

I maintain humidity for outdoor plans by draping a dry cleaner bag over them while they're indoors. Be sure to water and pay attention to the leaves... drooping means "water me!"

Your tree looks like a fiddle leaf fig to me. I have a humidifier on mine.