Every word of two or more syllables has one of them which carries the main stress of the word.
The vowel in that stressed syllable gets a little tonos mark above it.
So you use ά (alpha with tonos) if it is the vowel of a stressed syllable.
The letter λάμδα is pronounce "LAM-tha" with stress on the first syllable, so it's written λάμδα.
If the accent had been on the second syllable, lam-THA, then it would have been λαμδά.
Leaving off the accent completely, as in λαμδα, is just wrong -- the accents are part of the spelling of the word.
"λάμδα is the name of one of the letters in the Greek alphabet. It looks like this when used in a word: Λ or λ.
See here for more information about the Greek alphabet...and much more.
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It will also help you learn the alphabet and where to find other useful links.
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Every word of more than one syllable has a stressed syllable (which has to be one of the last three syllables).
That stressed syllable is, as in English, pronounced a bit more loudly. (Rather than, say, at a higher pitch.)
The vowel in that stressed syllable is marked with an accent (called tonos in Greek).
For example, the words νόμος "law" and νομός "nome, prefecture" are distinguished by the fact that the first word is accented on the first syllable (NO-mos) and the second word is accented on the second syllable (no-MOS).
Thus the spelling λάμδα shows that the word is stressed on the first syllable, because the first alpha has the tonos on it.
There is no difference in pronunciation (as in vowel quality or quantity) between a vowel with tonos and one without; the vowel is not pronounced longer or slightly differently (as with English "long e" in "beet" versus "short e" in "bet", or the like); the tonos just indicates the position of the word accent.
I presume this was a listening exercise -- we can't make alternatives for those and so only one version is accepted.
The good news is that individual letters should only have listening exercises for the spelled-out version of the name, so if you hear "λάμδα" then you know that you have to type "λάμδα" and not just "λ" (which would sound like "llllll" rather than like "lamtha" after all).
For little phrases such as το κακό τ, it's still ambiguous unfortunately.
This is something we hope to improve in the next version of the course.