Questions & Answers about Завтра мне нужно проверить один документ.
Why do we use the perfective verb проверить here instead of the imperfective проверять?
When using нужно (need to), the perfective aspect is used if we care about the completed result. You need this specific document checked and finished. If you were talking about the ongoing process of checking, or a repeating habit like "I need to check documents every Friday," you would use the imperfective проверять.
Why does the sentence use мне instead of я?
Words like нужно (necessary) and надо (need) create impersonal sentences. The person who feels the need is treated as the recipient of that need and is put in the Dative case. So я (I) becomes мне (to me). Literally, this is "Tomorrow to me it is necessary to check one document."
What case is один документ in here?
It is in the Accusative case because it is the direct object of the action (it is the thing you are checking). Because документ is a masculine inanimate (non-living) noun, its Accusative form looks exactly the same as its Nominative (dictionary) form. The number один changes to match it and also looks like its dictionary form.
Could I put завтра at the end of the sentence instead?
Yes, you could say Мне нужно проверить один документ завтра. Russian word order is flexible. Putting завтра at the beginning just sets the context right away, emphasizing that "tomorrow" is when this specific task needs to happen.