So why is it we do not read the letter instead of we did not read ... ?
The Swedish sentence is in the present tense: läser = reads. But we did not read is past tense, in Swedish that would be vi läste inte.
You're right, the vi sounds very blurry here. I've disabled the 'listen and type' exercise for this sentence now.
"We do not read the letter" sounds very strange in english. We did not read the letter would sound more natural. Because it's definitive, it sounds as for example, if you received a letter after someones death. We do not read THE letter (EVER) makes sense, but it wouldn't be a very commonly used sentence. We did not read the letter however, sounds less important. We didn't read it, but would/could later.
I'll try changing the main English translation to We are not reading the letter, hopefully that will make things clearer.
Unfortunately Duo has some automatic typo handling for English that we cannot change.
Out of curiosity, in English, this sentence ("we are not reading the letter") would be interpreted generally as "we are not going to read the letter" (i.e. it will never happen) rather than "we are not currently reading the letter" (i.e. it isn't happening at the moment).
Is this true in Swedish as well or no?
läser covers both. In general, the Swedish present covers both the English present and present continuous, and in some cases can be used when you'd prefer a futural construction in English.
As a general comment, you have a few translations into english which although not wrong do not sit right on the ear. I am reminded of a Churchill quote, "Up with which I will not put." Which he explained, while perfectly correct grammatically no englishman would ever express in this way.