The Spanish hay is a remainder of times when haber was a not only a modal verb when forming compound tenses but a full verb (to own). Hay nowadays has the meaning of there is/are. Hay has only one conjugation = hay.

hay = there is / there are Hay mucha gente en la calle. = There are many people on the street.

Since there is has also in some sense something to do with location there is the possibility to mix hay with estar. Actually sometimes you can use both sometimes not.

Hay zanahorias en el refrigerador. = There are carrots in the fridge. Las zanahorias están en el refrigerador. = The carrots are in the fridge.

If you think about the association that you get with the two sentences you'll see the difference clearly. The first sentence say there are carrots (some - nobody knows what kind, how much etc.) in the fridge - no article! The carrots in the second sentence have an definite article. So we know something about the carrots. The carrots (those I bought yesterday) are in the fridge. You wouldn't say The carrots there are in the fridge - wouldn't you? Therefore, the only possible translation is with estar.

The carrots I bought yesterday are in the fridge. correct: Las zanahorias que compré ayer están en el refrigerador. incorrect:Hay las zanahorias que compré ayer en el refrigerador.

Whenever you try to put something into Spanish that means there is then hay is the right choice. In other words: When describing something indefinite, something without an article hay is to be used.

Hay unos hombres en la puerta que preguntan por ti. = There are some men at the door that ask for you. Los hombres de al lado están en la puerta. = The men from next door are at the door and ask for you.

Había mucha gente en la calle. = There were many on the street. Ayer hubo un accidente en Madrid. = Yesterday there was an accident in Madrid.