What are the singular forms of haber?

The verb "haber" is already in its singular form. It doesn't have a plural form because it's a impersonal verb, meaning it doesn't have a subject.

However, "haber" can be conjugated with different tenses, which can be used to express different meanings. Here are some examples:

* Hay: This is the singular form of the verb "haber" in the present tense, meaning "there is" or "there are".

* Habrá: This is the singular form of the verb "haber" in the future tense, meaning "there will be".

* Hubo: This is the singular form of the verb "haber" in the preterite tense, meaning "there was" or "there were".

So, while "haber" itself doesn't have a singular or plural form, its conjugations can be used in singular or plural contexts depending on the tense.