Breakdown of Он мчался стрелой сквозь толпу, чтобы спасти друга.
он
he / it (for a masculine noun) / it (for a neuter noun)
друг
friend
чтобы
in order to
толпа
crowd
Questions & Answers about Он мчался стрелой сквозь толпу, чтобы спасти друга.
Why is the word стрелой in the instrumental case here instead of using как стрела (like an arrow)?
You have spotted the Metaphorical Instrumental of comparison. In Russian, you can compare an action to a noun by putting that noun in the instrumental case without a preposition. While как стрела is also perfectly correct and more common in everyday speech, using стрелой sounds more poetic and dynamic, emphasizing the physical manner of his movement.
Could we use бежал (ran) instead of мчался in this sentence?
You could, but you would lose the intensity. The verb мчаться implies moving at extremely high speed, often recklessly or headlong. Paired with the metaphor стрелой, мчался paints a much stronger picture of him dashing at top speed than the neutral verb бежать would.
Why does the sentence use сквозь толпу rather than через толпу? Both mean "through", right?
Yes, but they have different nuances. The preposition через often implies simply crossing a space from one side to another. Сквозь implies pushing through something dense that offers resistance, like a thick forest or a packed crowd. Because he is rushing like an arrow, сквозь captures the feeling of him physically piercing through that obstacle.
Why is the perfective infinitive спасти used right after чтобы? Do we not sometimes need the past tense after чтобы?