Quick question about network switches

Hi everyone!

Hope you can answer my question about a network switch. I'm planning on buying an 8-port gigabit switch because the router/switch/modem combo unit which we got from the ISP is running out of ports (4 isn't enough, sadly). Am I right in saying that I can use one of the ports for the switch and then the other 3 left can still be used like normal? Or does the combination unit have to go into some sort of "switch" mode where only the single port works?

Cheers,
Sam