After download a big app,I it says that there is insufficient space; and it stopped downloading. After this i checked application manager and it said that I had low space despite the app not downloading what do I do?
That's a very loose question. If your phone comes with an inbuilt File Manager, use that to navigate to the location and delete and residual files.
You could also use third party applications such as Clean Master to detect and remove unwanted files.
Try deleting the cache in Settings -> Storage. It probably holds the downloaded part and it could be already large from earlier as it contains other apps caches.
Try uninstalling high mb apps that you don't need anymore,also try using SD Maid to remove useless files.