This sentence I found in my Classical Greek textbook. I suppose that κατατιθου is a form of the verb κατατιθημι but I cannot explain the form. Is this an exceptional form or is it just a spelling error in my textbook?
This phrase is from a Greek-Latin Lexicon compiled by Thomas Morell, a British scholar of the 18th century. In this Lexicon each Greek entry is followed by: its Latin translation, a Greek exemplary phrase, ancient or hellenistic one, containing the entry word and synonyms. The entry word of your example is ἐφόδιον (=provision), whereas κατατίθου is supposed to be imperative [2nd person singular of the present tense] of the middle voice verb κατατίθεμαι (=put aside, store, save). I say "is supposed to be" and not "is", because such form never existed in Classical Greek [the AG present tense imperative form is "[κατα]τίθεσο". The easy conclusion is that the specific form of your example comes from the hellenistic period of the Greek language, when many changes had already made their appearance. So, it is not a spelling error in your textbook. The meaning of the phrase-philosophical saying is: "Always store provisions for the time you will be old".