Hey Ximena, how are you? I hope you are doing just fine!
1) "She insisted that we be at the office at three o'clock".
2) "it's important that he complete the presentation in less than thirty minutes".
In the examples you provided, you may find it wrong to say ''we be'' and ''he complete'' instead of the ordinary forms of these verbs (''are'' and ''completes''). I assure you that your worry is completely normal and also that there is nothing wrong about those sentences. Moreover, I shall provide you with an explanation about the Grammar Pattern used in the contexts given, which we call ''The Subjunctive Mood/Tense''.
* Expressing Commands, Suggestions, Requests, and Statements of Necessity:
When we express actions that we demand, suggest, or request that someone else take, or describe something that must be the case, we use the 'base form' of the verb — that is, the infinitive form without the word 'to'.
“He demanded that they leave the room at once.” (command)
“I recommend that she study harder next time.” (suggestion)
“I ask that the audience be completely silent during the demonstration.” (request)
“It’s necessary that we be vigilant to avoid another disaster.” (statement of necessity)
The biggest difference between the subjunctive and the indicative mood, in this case, is that the verb does not change according to who is taking the action. For instance, it is 'she study', 'the audience be', and 'we be' in the subjunctive, while it would be 'she studies', 'the audience is', and 'we are' in the indicative mood.
** Note that when we issue direct demands using imperative sentences (as in “Do your homework!” or “Please close the window!”), we are no longer using the subjunctive mood — instead, we are using what’s known as ''The Imperative Mood''.
Hope you have understood it! :)
Should you have any further doubts, do not hesitate to contact me. I shall be pleased to help!