"He would like thirty olives."
Translation:Triginta olivas velit.
olivas is the accusative plural. The accusative is often used as the direct object, the thing being directly affected/acted upon by the verb.
Olivas habent -> "They have olives"
olivae can be the genitive singular, dative singular, nominative plural, and vocative plural. I will only explain the nominative. It is used as the subject of the sentence, the person or thing that is 'performing' the verb.
Olivae habent -> "The olives have" (have something, I don't know)
This link mentions the difference between is (297.b) and ille (297.d).
It lists the two as:
Ille is used of what is remote (in time, etc.); and is hence called the demonstrative of the 3rd person. It is sometimes used to mean “the former”; also (usually following its noun) of what is famous or well-known; often (especially the neuter illud) to mean “the following.”
Is is a weaker demonstrative than the others and is especially common as a personal pronoun. It does not denote any special object, but refers to one just mentioned, or to be afterwards explained by a relative. Often it is merely a correlative to the relative quī.