Ongoing Processes vs. Completed Results
Modal words like надо (need to), нужно (need to), можно (can/may), and нельзя (cannot/must not) are followed by a verb in the infinitive.
For habits, ongoing processes, or constant actions (постоянно), use the imperfective aspect.
| You need to check these documents constantly. | Надо постоянно проверять эти документы. |
| You can't rest all the time. | Нельзя постоянно отдыхать. |
For a single, completed result or a specific one-time task, use the perfective aspect.
| I need to check one document tomorrow. | Завтра мне нужно проверить один документ. |
| You need to rest a little after work. | После работы тебе надо немного отдохнуть. |
Strong Prohibitions
To say an action is strictly forbidden ("must not"), use нельзя followed by an imperfective verb.
| You must not open this door! | Эту дверь нельзя открывать! |
Immediate Results and Sequences
To focus on an action being completed right away (сразу), use a perfective verb.
| But this mistake needs to be corrected right away. | Но эту ошибку нужно исправить сразу. |
Similarly, use perfective verbs when listing a sequence of chores to emphasize getting each task done.
| I need to clean the kitchen and wash the clothes right away. | Мне надо сразу убрать кухню и постирать бельё. |
| I need to check the text and correct the mistakes. | Мне нужно проверить текст и исправить ошибки. |
Lack of Necessity
To say that someone "does not have to" or "doesn't need to" do something, use не нужно or не надо with an imperfective verb.
| You don't have to correct these old mistakes. | Тебе не нужно исправлять эти старые ошибки. |
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