Olá, @roberta.a.page. It’s hard to help you make sense of these prepositions because they’re not always used in consistent ways.
Regarding the example you gave, first, you should know that for us, bar usually refers to the whole place, while balcão is the word we use for the counter where you can order drinks or sit/stand by. Then, you also need to keep in mind that in these contexts, ao is usually used either for movement or to say that you’re next to/by something, while no usually means that you’re in/on a place.
So, we can say things like:
- Ele toma uma bebida no bar -> He has a drink in the bar
- Ele toma uma bebida ao balcão -> He has a drink by the counter/next to the counter
- Ele vai ao bar -> He goes to the bar (movement)
- Ele está no bar -> He’s in the bar
But we can’t say:
- Ele toma uma bebida ao bar
- Ele vai no bar
- Ele está ao bar
Other examples with a/ao:
- Ela senta-se à mesa. -> She sits at the table (by the table)
- Nós estamos à janela. -> We’re at the window (by the window)
- Eu irei ao teu jantar. -> I’ll go to your dinner.
And with no:
- A comida está na mesa. -> The food is on the table.
- Há trânsito na estrada. -> There’s traffic on the road.
- O objeto está no saco. -> The object is in the bag.
@mac.cummings, anytime you see no/na/nos/nas, em is there. I suppose your question is more about when to add the articles that contract with it (e.g., em + o = no) and when to leave em by itself.
- When indicating locations or positions:
– With any common nouns, you’ll generally use the contracted forms. One usual exception: the noun casa. We often say “em casa” for “at home” (our house), and “na casa” for “in the house” (any house).
– With proper nouns, you’ll generally use the contracted forms when referring to people (i.e. their names) or specific places (e.g. the Museum of Fine Arts). For towns/cities/regions/countries, it will vary, as discussed here: Eu moro em, no, na?
Examples:
Eles estão na escola. (They’re at school)
O dinheiro está no bolso. (The money is in the pocket)
Estou em casa. (I’m home)
O implante está no João. (The implant is in John)
Nós estamos no Santuário de Fátima. (We’re at the Sanctuary of Fátima)
Ela vive em Cardiff, no País de Gales. (She lives in Cardiff, in Wales)
- When describing emotions or states: you’ll generally use em on its own, without any articles.
Examples:
Tu estás em choque. (You’re in shock)
Eu estou em stress. (I’m stressed)
Nós estamos em pulgas. (We’re very excited -> literally, like jumpy fleas)
- When indicating positions: you’ll use em for on top and down/under, but à (preposition a + definite article a) for left and right.
Examples:
O livro está em cima da mesa. (The book is on top of the table)
O vizinho vive lá em baixo, no rés-do-chão. (The neighbour lives down there, on the ground floor)
A televisão está à esquerda da lareira. (The TV is to the left of the fireplace)
O teu carro está à direita. (Your car is to the right)
- Other cases? Good luck! (can’t think of any other specific cases now)