I've got a HTC phone with 8gb built in which is full up but I have a memory card in the phone too which has 1gb left. Why can't I install more than 1 app at a time?! I always get a message saying not enough storage but I have enough with the memory card so why doesn't it download it to that?!
As Dementedpixie says you cannot move pre-installed apps. Some apps that you download also insist on being installed on device memory and can't be moved to the memory card.
You may need to figure out why your device memory is full. Under settings find Storage (assuming a recent version of Android) and it will give you a breakdown of how your memory is being used. If there is a lot of cached data tap on that and you will be given the option to clear it. That won't damage your phone or stop anything working. If you tap on "Used space" you will get a breakdown showing the space occupied by applications, pictures, audio and downloads which may also help you free up space. If you've got a lot of pictures you can save space by copying them to your PC and deleting them from your phone.