You can't download apps directly to the SD card if it isn't made to be "Internal" storage, which you said your phone can't do. The only other option you have is to clear out the phone of things not needed. Many apps save all data to the phone, so the info you don't need for any app you can remove by going into "Settings" then "Apps" and then tap on the app you wish to clear out and then tap either "Clear Cache" to clear only some of its data or tap "Clear Data" to clear all the data that app holds. Making the app like it was when first installed. Just be sure you save anything you want to either the card or a google account or a computer before you clear them as once it is gone, it is gone.
Your other option is to uninstall apps you don't really use.