"I need it."
Because Αυτό χρειάζομαι would have meaning of "This is what I need" rather than "I need it" in Greek. ^.^
How can I tell that I(!) need it (I is the subject)? Is "Χρειάζομαι" an exception/irregular?
Hi garpet! "Χρειάζομαι" is a deponent verb, i.e. a verb whose voice is active but looks like it's passive. Deponent verbs do not also have another form in active voice. Verbs in their passive voice forms, as well as deponent verbs, end in "-μαι".
You know that the subject is "I" because "χρειάζομαι" is first person singular. If the subject was second person singular, then you would see "χρειάζεσαι", if it was third person singular, you would see "χρειάζεται", etc. Look up conjugation of "χρειάζομαι" for more!
Because, as parents keep telling their children, there's a difference between "wanting something" and "needing something".
θέλω is to want, not to need.
Because that just means "I". It does not mean "I need it."
You need a verb and an object pronoun as well, e.g. Εγώ το χρειάζομαι.
Because that means "I need", not "I need it". You did not translate the word "it".
You can leave out subject pronouns in Greek, because those are obvious from the verb, but not object pronouns -- unlike (say) Klingon or Basque, those are not marked on the verb.