Flushing problems? Toilet that won’t stop running? Check out the chain that connects the flush lever to the flapper at the bottom of the tank.
If the chain is too short, it prevents the flapper from easily falling and completely closing the hole so the tank refills. Until you get a longer chain, unfasten the short one—it’s often clipped to an “arm” inside the tank—and re-attach it using a paperclip or a few paperclips, if need be, to add length. (Use plastic-coated paperclips if possible; they’ll last longer.) You need slack in the chain, but not so much that when you press down on the flush lever, the chain can’t get taut enough to lift the flapper, or lift the flapper fully.
If the chain is too long, you’ll have to hold the flush lever down hard to get a full flush instead of just briefly pressing on it. You can shorten the chain by adjusting which link is connected the clip, but any dangling excess can tangle and catch (and keep the flapper from re-settling normally) when you flush the toilet. You only need about an extra inch of chain; cut the rest off (use cutting pliers).
If the chain is rusty, replace it. If it breaks before you can do that, re-attach the two pieces with a paperclip, or, in a pinch, a twist tie. This won’t last long, but at least it’ll allow you to flush the toilet normally instead of having to reach into the tank.