I just moved into an apartment where the bathroom was recently renovated. However, the threshold was quite loose. When I inspected it, it appeared that that threshold had just never been attached and pulled loose without any resistance at all. The threshold is some hard, composite material. It is between the tile floor of the bathroom and the wood floor of the hall. I'm not sure how to attach the threshold to the ground underneath it. Any advice?
I normally set a threshold of that sort in a bed of thinset mortar, taping off the edge of the wood.
You could also use construction adhesive.
If a rental apartment, contact the landlord and advise that the tile people seem to have not finished the job properly, rather than fixing it yourself. If it's a place you own, do what you will.