In Ubuntu 16.04, there is an option in the Ethernet settings that called “method”, and I can choose “Shared to other computers” option. So I can use a cable to connect my raspberry Pi and my PC, and the raspberry Pi can use the network which is shared by the WiFi in PC.
But in Ubuntu 18.04, I can’t find the “method” option. So what can I do to solve this problem?
I would suggest seeking support on one of the many Support options available to you, on this page https://community.ubuntu.com/t/finding-help/712.
This site exists for co-ordination of the Ubuntu project, and not for support.