Learn Portuguese (Portugal): O caderno está debaixo da mesa.

The phrase for "under" is debaixo de. However, Portuguese combines de with the word for "the" that comes right after it. Since mesa (table) is feminine, it uses a (the). When you mix de + a, they contract to form da.

Why do we need the word de in debaixo de? Can I just say debaixo a mesa?

No, you always need de. In Portuguese, basic location words like debaixo (under), em cima (on top), and ao lado (next to) function a bit like saying "underneath of" or "on top of" in English. You must include de to connect the location to the object.

Why do we use está for "is" here instead of é?

The verb estar (which está comes from) is used for physical locations, even if the object doesn't move often. You would use ser (which é comes from) for permanent traits, like what color the notebook is, but estar is the strict rule for saying where things physically are.

I know mesa means table, but will "under" always use da?