Welcome to Android Central! Which ZTE phone? How much Internal Storage did it come with? If it was only 4 or 8 GB, that doesn't leave much for you to install apps, since about 3 GB is already taken up by the system. And you can't install apps directly to the SD card--they have to download and install to Internal Storage first, after which a portion of the app might be moveable to SD (if the phone supports it).