This game supports multiplayer online or lan?, or its only single player??

It's on the gamecard, game modes single-player.

Where do I put the health and stamina potions and how do I get them?

You put them in the bar to the right, and you get them either on item drops on the field or buying them.

Thanks. How do I get the item from the bar into the Paladin?

Press the hotkey associated with the slot to use the item. So if it's in slot 2, press 2.

...You know the game has a manual, right?

Thanks

Did not know about the manual

No problem. Look for 'Game Companion' in the game's bonus files if you've not found it already.

The Sheriff will recompense you for those.

Can't open locked doors or treasure boxes; can't give back daughter and can't walk through portal.

ad locked doors/treasure boxes: You can use Thieving skill, you can bash them open (switch to Attack mode with T and attack the door/chest, you can press Z then to order your companions to attack it as well), or you can use Shatter of Decay spells. Two notes though - firstly, if you open a chest in town with force, you will get apprehended by soldiers which means that only Thieveing can help you with them, and secondly, there are some locked doors that can be opened with correct keys only.

ad daughter: You mean Natalia Reichmann? If you have her along, talking to her father in Hillbrandt should be enough. If not, please send me your save file (from "Documents\Inquisitor_SaveGames") to kovar[@]cinemax.cz and I will have a look at it.

ad portal: You have to activate it from the other side first.