Hohl vs. leer
The German words hohl and leer both relate to the concept of emptiness, but they are used in completely different contexts. The word hohl refers to the physical structure of an object, meaning it has a cavity or is hollow on the inside. On the other hand, leer describes a temporary state of a container or space that currently contains nothing, meaning it is simply empty. Understanding whether you are talking about an object's permanent shape or its current contents is key to choosing the correct word.
Hohl
The word hohl means hollow and describes an object that has an empty space or cavity inside its structure. It is used when emptiness is a defining physical characteristic of the object itself, such as a pipe or a cave, and can also describe a hollow sound or shallow, meaningless words.
Der alte Baum im Garten ist innen völlig hohl.
(The old tree in the garden is completely hollow inside.)
Dieser Schokoladenhase ist hohl, was mich ein wenig enttäuscht hat.
(This chocolate bunny is hollow, which disappointed me a little.)
Vögel haben hohle Knochen, damit sie besser fliegen können.
(Birds have hollow bones so that they can fly better.)
Wenn man gegen die Wand klopft, klingt sie hohl.
(When you knock on the wall, it sounds hollow.)
Seine Versprechungen waren nur hohle Phrasen ohne echten Wert.
(His promises were just hollow phrases without real value.)
Das Rohr muss hohl sein, damit das Wasser hindurchfließen kann.
(The pipe must be hollow so that the water can flow through it.)
Leer
The word leer translates to empty and indicates that a space, container, or surface currently holds nothing or is unoccupied. Unlike hohl, the word leer refers to a temporary state rather than a structural feature, meaning the object is meant to be filled or used, such as a glass, a room, or a battery.
Mein Glas ist schon wieder leer, also brauche ich mehr Wasser.
(My glass is empty again, so I need more water.)
Die Straßen waren am frühen Sonntagmorgen völlig leer.
(The streets were completely empty on early Sunday morning.)
Ich kann dich nicht anrufen, weil mein Akku leer ist.
(I cannot call you because my battery is empty.)
Der Lehrer schaute in die leeren Gesichter der Schüler.
(The teacher looked into the empty faces of the students.)
Er hat den leeren Karton in den Müll geworfen.
(He threw the empty cardboard box into the trash.)
Das Haus steht seit vielen Jahren leer und verfällt langsam.
(The house has been standing empty for many years and is slowly decaying.)
Summary
In short, use hohl when you want to say that an object is structurally hollow, like a pipe or a chocolate figure. Use leer when you want to express that a container or space is currently empty and holds nothing, like a glass of water or an unoccupied room.