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Many people have given answers, but they lack a bit of explanation.
The short answer to your question is that seven is both εφτά and επτά. Both are acceptable and accepted.
The longer answer is that εφτά comes originally from the Demotic language, i.e. the spoken, informal language, while επτά comes from the Puristic language, i.e. the written, formal language. Because of the history of the Greek language, this has two consequences:
- While it's accepted, using επτά has a formal, old-fashioned feel to it. Using it in speech sounds very affected.
- Using εφτά is a bit more informal, but in speech it's the word you'll hear most often, 95% of the time. So if you want to remember only one form, remember this one.
The story is exactly the same for eight (informal οχτώ vs. formal οκτώ) and nine (informal εννιά vs. formal εννέα). In all cases the informal form is used much more often than the formal one, which is considered old-fashioned, although it's still known and understood.
Note that the situation is the same for the numbers 17 (δεκαεφτά vs. δεκαεπτά), 18 (δεκαοχτώ vs. δεκαοκτώ) and 19 (δεκαεννιά vs. δεκαεννέα), and for the numbers 700 (εφτακόσια vs. επτακόσια) and 800 (οχτακόσια vs. οκτακόσια). The only exception is 900, which has only one form considered correct: εννιακόσια. "Εννεακόσια" can be found, but is frowned upon. |