The general rule for Russian word order is to put new information at the end of the sentence. Thus, in answer to the question Ктћ предстђвил Сђшу? the word Мђша (the new information) is saved till the end of the sentence: Сђшу предстђвила Мђша 'Sasha was introduced by Masha.'
Another way of stating this rule is: an element that could serve alone as a short response comes last when part of a long response:
Ктћ предстђвил Сђшу? — Мђша. (short answer)
Ктћ предстђвил Сђшу? — Сђшу предстђвила Мђша. (short answer comes last)
Sentences like this, where the Nominative subject (Мђша) comes at the tail end of the sentence, can be translated with the English passive construction, using the word by: 'Sasha was introduced by Masha'. The Russian construction is called the pseudo-passive.
In spoken Russian you can get the same effect by accentuating the subject in initial position, e.g. МЂША предстђвила Сђшу, but in written Russian that auditory option is not available, so the pseudo-passive construction (Object+Verb+Subject) is very commonly used.