"She needs a coat."
Translation:Ela precisa de um casaco.
"Ela precisa dum casaco" is perfectly fine in BP (Brazilian Portuguese). But we don't use the contractions from the indefinite articles in formal/written language. So you can say "duma, dum", but you can't write them, you must write "de uma, de um". Concerning contractions from definite articles, all of them are accepted as far as I know, the only exceptions are "para o, para a" (you can say "pro, pra", but not write). I hope this helps! =)
I simply cannot see the logic of Ela precisa de um casaco, "She needs the a coat" I cannot detect a logical pattern for when to insert "The" (a specific) into a sentence saying she needs "A" (non-specific item.)
Can someone enlighten me in simple terms why and when to insert De?