Do I need a comma in the phrase "In your letter you've asked me about my relationship with music."? Grammarly (a Chrome add-on that checks spelling and punctuation) corrects it, inserting a comma after the word "letter", but it's not an introductory word/phrase, so either I misunderstand something, or Grammarly is wrong.
What's the case?
Why do you think it isn't an introductory phase? It's a prepositional phrase that sets up the context for the main clause. I always use a comma, but some style guides recommend against one for short phrases. For example, Purdue's OWL site says,
Some introductory elements don't require a comma, and sometimes the subject of a sentence looks like an introductory element but isn't. Do not use a comma in the following cases:
After a brief prepositional phrase. (Is it a single phrase of fewer than five words?)
After a restrictive (essential) appositive phrase. (See our document on appositives.)
To separate the subject from the predicate. (See below.)
https://owl.purdue.edu/owl/general_writing/punctuation/commas/commas_after_introductions.html
