Hi Rachelyeo,
The trick here is the word that comes after in, alto. In means in, but in alto means above, in the upper area!
Su is generally used with movement (Voglio andare su - I want to go up) and for translating "on", as in on the table, sul tavolo, on the wall, sul muro.
In alto is generally used to point at a general location. Sono in alto. I am above.Sono su. There is no "anchor point" as a table or the wall. Because of this it's also common to say sentences like Le chiavi sono in alto sul muro, the keys are above, on the wall.
Hope this helps!
Lucia
The trick here is the word that comes after in, alto. In means in, but in alto means above, in the upper area!
Su is generally used with movement (Voglio andare su - I want to go up) and for translating "on", as in on the table, sul tavolo, on the wall, sul muro.
In alto is generally used to point at a general location. Sono in alto. I am above.
Hope this helps!
Lucia