Fore ground, back ground building?

[Archetype]

So I'm new to Terraria. I tried to build a small house, but when I try to put blocks in the fore ground, it doesn't work, but I can only place blocks in the background. Randomly it will let me place blocks in the foreground, but I don't know what triggers it. So What do I do?

[dino]

you need to hammer out the dirt walls then put in your wall

[Archetype]

Oh ok thanks you. Noob question I know :)

[Celator]

Ordinary blocks will always be placed in the foreground; to put them in the background, you have to craft them into walls at a Work Bench. Also, like Duke said, you can't place walls on top of existing walls. Walls can only be removed with a hammer, while blocks can only be removed with a pickaxe.