The storage in my phone (it's a Huawei Honor 4C) is already full, so I inserted a micro SD card into it that has 8 GB available space. I tried many times to download applications but it didn't. What is the problem?
Just inserting a SD card didn't mean that you can install apps in a phone which is already occupied a maximum of internal.
You have to change the default install location in settings. Which is not available in many devices.
Go to apps navigate to all apps, click on the app name to access details.
Choose Move to SD card. So the app will be moved to sd.
Likewise, make some space in your internal for the new apps..
Also move files like, songs, videos , etc. To sd.
If your phone has the option enable to install apps on SD card, go for it.