Every once in a while when you have mastered some grammatical backflip, you discover that one is not enough: you have to do a grammatical double gainer. This is the case to say that someone “doesn't have” something or that “there isn't” something somewhere.

You recall that when a person has something, in Russian we express by saying “At so-and-so is such-and-such.” The person shows up in the genitive case as the object of the preposition у, and the thing the person has shows up in the nominative case, and the “there is/are” is expressed by the word есть:

When the person doesn't have something, the “doesn't have” idea is expressed in the present tense by нет, and the thing he doesn't have shows up in the genitive case.

The past tense of нет is always не было, and the future tense is не будет. Notice the contrast between these sentences. In the “have” sentences, the verb changes according to the gender/number of the subject. In the “doesn't have” sentences, the verb does not change for the subject because there is no grammatical subject (that is, nothing in the nominative case), and the verb defaults to neuter singular form:

These tricks also apply to sentences expressing sentences with the idea of “there is no” or “there was no”: