Why does the Internet WI-FI connection of my laptop disconnect when I connect the ethernet cable to the LAN?

These are two separate networks, the LAN is without Internet, I just use it to connect the tablet to the laptop and smart phone in my home network.

The WI-FI is owned by the landlady and connects to the Internet.

I know it should be possible to be connected both to the Internet through WI-FI and to my other computers and gadgets through the LAN because sometimes it works, but sometimes when I connect the ethernet cable the WI-FI disconnects and I wonder why?

Pongy

OS: Windows 7