There are different letters in Spanish with a similar pronunciation, and it makes difficult to decide whether we should write a word with “V” or  “B”, for example. The problem is that sometimes the misspelling can also imply a new word with a different meaning.
Today, I want to give you some hints to distinguish between some of these knotty words: haya and halla. Both of them are verbal forms, but the first one is much more frequently used than the second.

1. Haya, with y, is a form of verb haber:
El fabricante niega que haya desperfectos.
(The manufacturer denies that there are damages.)

Trick: if we can replace haya with pueda haber we can be sure that the correct form is with y.

El fabricante niega que pueda haber desperfectos.

2. Haya is also the name of a tree (beech).
Hay un bosque de hayas cerca de casa.
(There is a beech wood close to home.)

3. Halla with ll is a form of verb hallar. It is a synonym of encontrar (to find), so a good way to know if we have to write ll instead of y in the verb is to change it for encontrar and check the sentence´s meaning.
Halla muerto a su vecino en el ascensor.
(He finds his neighbor dead in the lift.)
Encuentra muerto a su vecino en el ascensor.

To be sure this verb is not with y, let´s apply the trick:
Pueda haber muerto a su vecino. (Nonsense)

We will find some exceptions to the rule obviously, but I hope this short explanation helps you with your doubts. And now, let´s practice with some exercises!

a) Ninguno de los entrevistados cree que la situación ________ empeorado.
b) Los muebles de mi dormitorio son de __________.
c)___________ la solución a este problema.
d) ¿Dónde se __________ la sección infantil?
e) Ojalá no _____________ más problemas.