Bem-vindo vs. boas-vindas
The Portuguese words bem-vindo and boas-vindas both relate to the English concept of welcome, but they serve different grammatical roles. The word bem-vindo acts as an adjective and a direct greeting, while boas-vindas functions as a noun representing the actual reception or the act of welcoming someone.
Bem-vindo
The word bem-vindo is an adjective used directly as a greeting to tell someone they are welcome. Because it is an adjective, its ending must change to agree in gender and number with the person or people arriving, taking the specific forms bem-vindo for a man, bem-vinda for a woman, bem-vindos for a mixed group or a group of men, and bem-vindas for a group of women.
Seja muito bem-vindo à nossa casa.
(Be very welcome to our house.)
Olá, bem-vinda ao seu novo emprego.
(Hello, welcome to your new job.)
Vocês são sempre bem-vindos aqui.
(You are always welcome here.)
As alunas novas foram bem-vindas pela professora.
(The new female students were welcomed by the teacher.)
Bem-vindos ao Brasil, esperamos que aproveitem a viagem.
(Welcome to Brazil, we hope you enjoy the trip.)
Boas-vindas
The word boas-vindas is a feminine plural noun that translates to the act of welcoming or a welcome reception. Unlike the adjective form, it never changes its spelling based on who is arriving, and it is frequently combined with verbs like dar to create the common expression dar as boas-vindas, which means to welcome someone.
O diretor deu as boas-vindas aos funcionários.
(The director welcomed the employees.)
Nós preparamos uma grande festa de boas-vindas para você.
(We prepared a big welcome party for you.)
Muito obrigado pelas boas-vindas calorosas nesta manhã.
(Thank you very much for the warm welcome this morning.)
O comitê de boas-vindas está esperando no aeroporto.
(The welcoming committee is waiting at the airport.)
Gostaria de dar as boas-vindas a todos os presentes.
(I would like to welcome everyone present.)
Summary
In short, use bem-vindo when you want to directly greet someone and say welcome, making sure to adjust its ending for the gender and number of the person arriving. Conversely, use boas-vindas when you need a noun to describe the welcome itself, such as mentioning a welcome party or the act of giving a welcome.