Haber is one of the more unusual verbs in Spanish. It may be the only verb that has a conjugation that varies with its meaning in a sentence. It is used primarily as an auxiliary verb (a verb used in conjunction with other verbs), but it can stand alone as a verb that does little more than indicate mere existence of the subject of the sentence. It can take a singular form even when used in the plural. It is also extremely irregular.
In this lesson, we focus on the haber usage that is usually the first one learned by Spanish students: as a vague verb usually translated as "there is" or "there are." In other lessons, we also look at the few idioms that use haber as well as its use as an auxiliary verb.
Standing alone, haber in the third-person present tense is usually translated as "there is" or "there are." Interestingly, though, the verb takes the form of hay (pronounced basically the same as "eye" in English) in both the singular and plural forms. Here are a couple of sample sentences: Hay muchos libros; there are many books. Hay un hombre en la sala; there is a man in the hall. (When using haber as an auxiliary in the third-person singular, ha or han is used, not hay. Han leído el libro, they have read the book.)
Haber can also be used in the same way for other tenses. The general rule for the other tenses is that the singular form is used for both singular and plural objects, although it is quite common, particularly in parts of Latin America, to use the plural forms with use of plural objects. Había muchas personas en la clase, or habían muchas personas en la clase, there were many people in the class. (In some areas, habían is considered substandard, so avoid it unless you hear native speakers using it.) Habrá mucho tráfico, there will be much traffic. No habría tiempo, there wouldn't be time. Quiero que haya tiempo, I hope there will be time.
Note that the "there" in these English sentences does not refer to location (grammatically, it is considered to be an introductory pronoun). When "there" refers to location, normally it would be translated using ahí or allí (or, less commonly, allá). Example: Hay una mosca en la sopa; there is a fly (a fly exists) in the soup. Allí está una mosca [said while pointing or indicating direction]; there (or over there) is a fly.
Note that other Spanish words or idioms can also be translated as "there + to be" in various usages. No queda queso, there is no cheese left (or no cheese remains). Seremos seis para el desayuno, there will be six of us for breakfast (literally, we will be six for breakfast). ¡Ahí viene el taxi! There's the taxi! (literally, there comes the taxi!) Esto provocó mucho llorar, there was much crying at this (or, this caused much crying). In some of these cases, the thought also could be expressed using a form of haber: No hay queso, there is no cheese. As in all cases, you should strive to translate for meaning rather than word for word.
Haber does not exist in an imperative form.