Can't put downloaded apps in my SD card
It's not always possible to move an app to an external SD Card via the
  Application Manager.


There is a feature, called Flex Storage, that allows you to use an external SD Card as Internal storage. This can be done from Settings > Storage > SD Card, depending to your phone model. Remember that usually this operation formats your SD, so make sure to make a backup first. 
By the way, this option is not supported from all devices.

Another way to achieve the result is using a third-party software like Link2SD that allows you to move the apllications to a special partition on your SD Card and use them as usual. Keep in mind that this operation requires Root Privileges on your phone.

There are plenty of guides on the internet about this procedure, by the way I suggest you to perform it only if you are sure, because an error while executing the procedure (especially when partitioning the SD) can lead to data loss.