Well you can only save directly to the SD card things like apps, and download directly to it, if the tablet is running "Marshmallow". If you are running any other Android version, you can't do it.
You can, however, move things like photos, videos, music, etc., over to the card. You can also set some apps to save things like new photos, etc., on the SD card. This will free up a lot of space for you to add any apps to the phones internal storage.