systemctl

Syncthing is one of the best tools for keeping folders synchronized across the internet. However, for one or the other reason, it occasionally stops to work on my Ubuntu 16.04 and because I use it in a "set it and forget it" fashion, I also forget how to check that it is operational.
Make it start automatically at boot time (as a system service): systemctl enable syncthing@username.service
Start the service: systemctl start syncthing@username.service

OpenVPN
sudo systemctl start openvpn@client
The "client" is derived from the configuration file name (/etc/openvpn/client.conf). If your configuration file is server.conf, the command needs to be
sudo systemctl start openvpn@server
When you want the service to start up automatically during system boot, you issue:
sudo systemctl enable openvpn@server
Templated versus non-templated services