Lago vs. lagoa
In Portuguese, both lago and lagoa refer to bodies of water surrounded by land. The primary differences lie in their size, depth, and sometimes their proximity to the sea. Generally, lago is the standard term for a large lake, while lagoa implies something smaller, shallower, or specifically a coastal lagoon.
Lago
A2This word corresponds to the English lake. It refers to a large, extensive body of water, usually freshwater, that is surrounded by land. A lago is typically deep and permanent.
O barco navegou calmamente pelo centro do lago.
(The boat sailed calmly through the center of the lake.)
Dizem que este lago é o mais profundo da região.
(They say this lake is the deepest in the region.)
Nós alugamos uma cabana com vista para o lago.
(We rented a cabin with a view of the lake.)
A superfície do lago congelou durante o inverno rigoroso.
(The surface of the lake froze during the harsh winter.)
Lagoa
B1This word is often translated as pond, small lake, or lagoon. A lagoa is generally smaller and shallower than a lago. It can refer to a small natural pond in a field, a decorative man-made water feature in a park, or a body of saltwater separated from the ocean by sand.
Os patos nadam tranquilamente na lagoa do parque.
(The ducks swim peacefully in the park's pond.)
Havia uma pequena lagoa cheia de sapos perto da fazenda.
(There was a small pond full of frogs near the farm.)
A água da lagoa é muito rasa para mergulhar.
(The water in the pond is too shallow for diving.)
Depois da chuva forte, formou-se uma lagoa no jardim.
(After the heavy rain, a pond formed in the garden.)
Summary
To choose the correct word, consider the scale: use lago for vast, deep bodies of water similar to the Great Lakes. Use lagoa for smaller, shallower bodies of water like a duck pond, a watering hole for animals, or a coastal lagoon.