Learn Russian: Иван вчера не был в офисе.

In the past tense, the verb быть (to be) changes to match the gender of the subject. Since Иван is a masculine person, we use the masculine form был. If the subject were a woman, it would be была.

Does не always go right before the verb?

Yes, when you are negating the action itself (saying he "was not"), the negative word не must sit directly in front of the verb it negates. This gives us не был.

How do you pronounce не был? Does the emphasis change?

Yes, it does! A quirky rule about the past tense of this verb is that the stress jumps to the negative word for the masculine, neuter, and plural forms. So you pronounce it НЕ был (emphasizing the first word).

Why is there an "e" at the end of офисе?

This is the Prepositional case, which you use to talk about physical locations. The dictionary form for "office" is офис. Because Ivan is in the office (after the preposition в), we add an -e to the end of the word to make it в офисе.