Complete the following sentences with the correct form and tense of the verb in subjuctive or the conditional of “haber.”

es raro que ellos no (hacer) los preparativos para la fiesta.

The correct answer is:

Es raro que ellos no hicieron los preparativos para la fiesta.

It's unusual that they did not do the preparations for the party.

First, we need to know with which person we are working, in order to conjugate the verb that we are using "Haber" (to have)

The Spanish person we have here is: They (Ellos)

It's a sentence talking about an action that "they" haven't done.

The correct conjugation for the verb "Haber" for this particular case is "hicieron"

Have a nice day!

google translate is the way to go, sorry i cant you but its a trusted resource to use