Learn Russian: Рассчитай ты траекторию вчера, мы бы уже запустили ракету. - Had you calculated the trajectory yesterday, we would have already launched the rocket.

This is a special conditional structure. By combining an imperative verb with a subject pronoun directly after it (Рассчитай ты), you create an 'if' clause. Even though it looks like a command, the context and the conditional second half (мы бы...) tell us it means 'If you had calculated.'

Could I use a regular 'if' clause with если to say this instead?

Absolutely. You could say Если бы ты рассчитал траекторию вчера... The imperative structure used here is just a more expressive, slightly colloquial alternative that means the exact same thing. Both require the second half of the sentence to use the conditional (бы + past tense).

Why is the pronoun ты required here, and why does it come after the verb?

Normal commands usually drop the pronoun (e.g., just Рассчитай! for 'Calculate!'). In this 'imperative as conditional' structure, including the pronoun and putting it immediately after the verb is exactly what signals to the listener that this is a hypothetical scenario and not an actual order.

Why use the perfective verb рассчитай rather than the imperfective рассчитывай?

The sentence focuses on the successful completion of the action: getting the trajectory fully calculated. Using the perfective emphasizes that the calculation needed to be finished for the rocket to launch. The imperfective would sound like 'If you had just spent time calculating.'