Learn Russian: Нам тяжело, но мы начали печатать документы. - It is hard for us, but we began to print the documents.

This is a strict rule in Russian: verbs that show the start, continuation, or end of an action (phase verbs) must always be followed by an imperfective infinitive. The action of beginning is a completed step (начали is perfective), but the printing itself is viewed as an ongoing process, so you must use the imperfective печатать.

Why does the sentence start with нам тяжело instead of using мы?

To express how someone experiences a situation (like it being hard, cold, or fun), Russian uses an impersonal construction. The person experiencing it takes the Dative case (нам for us), followed by the adverb form (тяжело). If you said мы тяжелые, it would literally mean we are physically heavy!

Why did we use the perfective начали instead of the imperfective начинали?

We use the perfective начали because the act of starting is a single, successfully completed event in the past. If you used the imperfective начинали, it would mean you were repeatedly starting to print, or were just in the middle of trying to start.

Is документы in the Nominative or Accusative case here?

It is in the Accusative case because it is the direct object of the action (the things being printed). For inanimate masculine nouns like документ, the Accusative plural looks exactly the same as the Nominative plural.