"You should eat at least an apple a day."
Translation:Εσύ πρέπει να τρως τουλάχιστον ένα μήλο την ημέρα.
It is. Both words mean the same thing. It's just that, sometimes, when the previous word ends in a vowel like η or α, μέρα is preferred, so that the two vowels (η/α and the η in ημέρα) don't clash.
It's the same story for ημέρα. When the previous word ends in a consonant (usually ν), it sounds a bit more neat to use ημέρα than μέρα. ^.^
The usage of the definite article here, both in Greek and English, specifies that there are more things you'd have to do, but at least eat the apple. For example:
My mother: You have to eat an apple, an orange and a pear every day.
Me: I don't want all these
Mother: At least eat the apple every day.
A basic question: what is the conjugation of πρέπει?
It's an impersonal verb, only used in the third person singular -- present tense πρέπει, past tense έπρεπε.
It may help to think of it as "it is necessary that..."
So πρέπει να τρως "you have to eat" is "it is necessary that you eat".
I translated it with the available tiles and was surprised that it was marked as correct. Θα was not among the tiles. However, in another post by Troll, who is quite erudite in both languages, it was stated that Πρέπει means MUST and θα πρέπει means SHOULD. The former is an imperative while the latter only a strong suggestion. Is there no consensus on this verb among Greek speakers?
You are right, Πρέπει/έπρεπε is must (present/past) and Θα πρέπει/θα έπρεπε is should (present/past). Πρέπει is strong whereas Θα πρέπει is a suggestion. I don't think that there is any disagreement among natives, it is probably a small mistake done after working a lot of time on translations. I changed the best translation to match this.