According to On Learning Brazilian Portuguese, assim is best translated as "in this manner". Lets test it (using random sentences from Tatoeba).
Random sentence 1
Meu irmão é muito importante. Ao menos ele pensa assim.
English translation of assim: "that he his" or "that this is true"
Random sentence 2
Ele deve estar muito bravo para dizer algo assim.
English translation of assim: "such a thing [in this way]"
Random sentence 3
Ela gostou do Charles assim que o conheceu.
English translation of assim: "as soon as"
In my mind, only example sentence 2 met the general "in this manner" definition of assim. However, it is useful to point out that when "assim" is used with the preposition "que", that the phrase (assim + que) takes on the meaning "as soon as". Examples:
"Quero que você me escreva assim que você chegar lá." - I want you to write me as soon as you get there
"Virei assim que possível" - I will come as soon as possible
"Partiremos assim que estiver pronto" - We will leave as soon as possible
"Eu te ligarei assim que puder" - I will call as soon as I can
When "assim" is preceded by a translation of the verb "ser", "[ser] + assim" takes a construction of "that way" or "in this way" or "in xyz manner" something happened. (poor explanation I know)
"Foi assim que ele fracassou" - This is how he failed
"Não foi assim que aconteceu" - Thats not how it turned out
"É assim que costumo preparar peixe" - This is usually how I cook fish
"Entendo, mas as coisas são assim" - I understand, but this is how things are
"E assim que eu fiz" - This is how I made it
To get further help, I went to the WordReference forums. Here's a few more great examples:
"Olha, é assim" - Look, it's like this
"Eu faço assim" - I do it like this
"É bom assim" - It's good like that
"Assim não dá" - It doesn't work like that / that way
Hope that helps. I can tell this word will require some practice to get the loose hang of it. Here's a Brazilian podclass that also addresses "assim".
