Breakdown of Я только что прочитал это письмо.
это
this / it is
я
I
письмо
letter
прочитать
to read (perfective)
Questions & Answers about Я только что прочитал это письмо.
The current lesson is about как только (as soon as), but here we see только что. Are they interchangeable?
No, they have different meanings. Как только means "as soon as" and links two actions together, which is the main focus of your current lesson. However, только что (used in this sentence) means "just" or "just now." It describes a single action that was completed very recently.
Why does the English translation use "have read" (present perfect) while the Russian verb прочитал is just in the past tense?
Russian does not have a present perfect tense. Because the action of reading the letter is fully complete, Russian simply uses the past tense of a perfective verb (прочитал). Whenever you want to say you "have just done" something, you will usually pair только что with a past tense perfective verb.
What case is это письмо in, and why didn't its ending change?
It is in the accusative case because it is the direct object receiving the action of the verb (it is what was read). You do not see a changed ending because neuter nouns like письмо (and their modifiers like это) look exactly the same in the accusative case as they do in the dictionary (nominative) case.
Can I change the word order and say Я прочитал это письмо только что?
Yes, but it changes the emphasis. In Russian, adverbs of time usually go right before the verb they modify, making Я только что прочитал the most neutral and natural phrasing. Moving только что to the very end places heavy emphasis on the time, as if correcting someone: "I read it JUST NOW, not yesterday."