Sentences
hay
One word for there is and there are.
hay never changes for number, so hay un banco and hay dos bancos both use it. It states that something exists, which is different from saying where a known thing is, and that job belongs to estar.
Examples
hay un banco por aquíthere is a bank around here
hay dos mesas libresthere are two free tablesstill hay, never hayn
¿dónde está el banco?where is the bank?estar, because you already know it exists
no hay problemathere is no problem
The mistake this causes
Making hay plural. It has no plural form in the present.