Breakdown of Надо постоянно проверять эти документы.
этот
this [f: эта, n: это]
документ
document
надо
must, need to
постоянно
constantly
Questions & Answers about Надо постоянно проверять эти документы.
Why do we use the imperfective verb проверять here instead of the perfective проверить?
The key is the word постоянно (constantly). It tells us this is a repeated, ongoing action. After modal words like надо (it is necessary), we use the imperfective infinitive when talking about processes or habits, and the perfective infinitive when we want a one-time completed result.
Who is doing the action in this sentence? There is no word for "you" or "I".
This is an impersonal sentence. By using just надо plus an infinitive, it means "one needs to" or "you need to" in a general sense. If you wanted to specify who needs to do it, you would add a pronoun in the dative case at the beginning, like Мне надо (I need to) or Вам надо (You need to).
What case is эти документы in?
It is in the accusative case because it is the direct object receiving the action of the verb проверять (to check). For inanimate plural nouns like документы, the accusative form looks exactly the same as the nominative plural. The pronoun эти (these) matches this form.
Could I put the word постоянно at the end of the sentence?
Yes, because Russian word order is flexible, you could say Надо проверять эти документы постоянно. However, putting the adverb right before the verb it describes is the most neutral and natural word order. Moving it to the very end would place extra emphasis on the fact that it must be done constantly.