Olá! I pronounce “am” in words like ão, and it sounds different in different words. For example precisam ,falam . Are there any rules I can apply to when I pronounce it? Many thanks.
The main thing to remember is that you don’t pronounce the “m” like you would if you were to read it like an English word. Instead, that -am ending is replaced with a nasal vowel. To get the nasal sound to come through it’s almost as you were are about to say -am with an m sound, but you just don’t actually close your lips at the end. (The air is directed through your mouth AND nose.)
Here’s a pronunciation guide that might be helpful - scroll down to the m and listen to the examples listed.
This Shorty also uses a lot of words ending in m to give you some more examples: Aula de Edição de Imagem