Questions & Answers about Jeg fik et brev fra min mor i går.
Could I start the sentence with i går to emphasize when it happened?
Yes, it is very common to put time expressions at the beginning in Danish. Just remember the V2 rule, which requires the verb to be the second grammatical element. If you move it to the front, the subject and verb must swap places: I går fik jeg et brev fra min mor.
Does fik in this sentence mean I actively went out and picked up the letter?
No, fik (the past tense of at få) strictly means 'received' here. It implies the letter was delivered to you. If you meant that you actively went to the post office to retrieve it, you would use hentede (fetched/picked up) instead of fik.
Why is it min mor when the other noun in the sentence, et brev, is neuter?
Possessive pronouns only agree with the noun they directly modify. Because mor is a common gender noun (en mor), you use min. The gender of et brev elsewhere in the sentence does not affect it. If you were talking about 'my letter', it would be mit brev.
Can I use af instead of fra to say the letter is from my mother?
You should use fra here. While af can sometimes translate to 'from' or 'by' (such as in passive sentences or to describe what something is made of), fra is the correct choice when indicating the sender or the physical origin of an object.