Terra vs. mundo
While both Terra and mundo can be translated as Earth or world, they are not interchangeable in Portuguese. Terra refers to the physical aspects, like the planet or the ground, while mundo refers to the more abstract or societal concept of the world.
Terra
A2Terra most often refers to the physical planet we live on, the ground beneath our feet, soil, or a piece of land. When capitalized (Terra), it specifically means the planet Earth. In a broader sense, it can also mean homeland or native land.
A Terra é o terceiro planeta a contar do Sol.
(Earth is the third planet from the Sun.)
Depois de meses no mar, o marinheiro finalmente avistou terra.
(After months at sea, the sailor finally spotted land.)
As crianças estavam a brincar na terra do jardim.
(The children were playing in the garden soil.)
Ele voltou à sua terra natal depois de muitos anos.
(He returned to his homeland after many years.)
Preciso de comprar um saco de terra para as minhas plantas.
(I need to buy a bag of soil for my plants.)
Apesar da fama, ele é uma pessoa com os pés bem assentes na terra.
(Despite his fame, he is a very down-to-earth person.)
Mundo
A2Mundo refers to the world in a more abstract, conceptual, or societal sense. It encompasses humanity, society, global affairs, or a specific domain of life or activity. It's about the planet as a place of human experience.
Ele quer viajar e conhecer o mundo inteiro.
(He wants to travel and see the whole world.)
O que é que o mundo vai pensar de nós?
(What will the world think of us?)
Ela é uma especialista famosa no mundo da moda.
(She is a famous expert in the world of fashion.)
Tenho um mundo de coisas para fazer hoje.
(I have a world of things to do today.)
As notícias do mundo estão cada vez mais complicadas.
(The world news is getting more and more complicated.)
No mundo dos negócios, é preciso ser competitivo.
(In the business world, you need to be competitive.)
Summary
In short, the key difference is physical versus conceptual. Use Terra when you're talking about the planet as a celestial body, the ground, soil, or a piece of land. Use mundo when you're referring to the world in terms of its people, societies, cultures, global systems, or a specific sphere of interest like the world of sports or the animal world.