Is the Hlavní nádraží metro stop

Hlani Nadrazi is the main train station and there is a metro stop inside the station (same name) near the main entrance /exit door,just head up or down, very easy and very obvious.
remember to get a ticket and stamp it in the yellow boxes at the entrance to the metro area, not doing this can lead to a 800kc fine. Tickets available from the ticket counter , the various machines and the Relay shop in the station.

Unclegus already explained everything. I would just add that Main Train Station Metro Station (Hlavni nadrazi) unlike most other stations which have one platform Hlavni nadrazi has 2 platforms. One platform is for trains going north, direction Letnany, the other for trains going south, direction Haje. If you know which direction you are going, choose the entrance appropriately because platforms are not connected. If you choose wrong one you would have to go back up to Main Train Station and take the correct entrance to your desired platform. Hlavni nadrazi is on red line C. If your station is on green or yellow line you will have to change trains in Muzeum or Florenc.

I remember in 2012 exiting the train platform to the front lobby where there was an ATM and a Metro ticket automat. I got Koruna, bought my Metro ticket, and went down stairs to the Metro station platform.

Thank you for posting, Lee. We're going to Prague in May and I'd been wondering whether I should get korunas ahead of time. Sounds like I don't need to do that.

thank you all for the above information. it is much appreciated