Learn Icelandic: Horfðu upp í loftið!

Upp is used because your gaze is moving upwards. In Icelandic, upp shows motion or direction toward a place, while uppi is used when something is already stationary in a high place (like being 'upstairs'). Since you are directing your eyes up toward the ceiling, we use the motion word.

I learned that 'to look at' is horfa á. Why isn't there an á in this sentence?

You usually use horfa á when looking at a specific object, like a screen or a person. However, when you describe looking in a specific direction or into a wide space, Icelandic often drops á and uses directional words instead. Here, upp í (up into) describes the path your gaze is taking, so á is not needed.

Why is the word for ceiling (loftið) in the accusative case here?

The preposition í takes the accusative case when it describes motion or direction towards a destination (in this case, your gaze traveling to the ceiling). If a spider were just resting stationary on the ceiling, í would trigger the dative case instead (loftinu).

What form of the verb is horfðu?