It's like English.
You just learn that sometimes you use the hard c, as in candy or camera, and sometimes you use the k, as in kangaroo or kindness.
Sometimes you use the u as in lucid and lucrative, and sometimes you use the diphthong oo as in loosen and noose.
Sometimes the /z/ sound is spelled with an s, as in rose or mosey, and sometimes you actually use a z, as in dozen or brazen.
This is a bit off topic but the reason English can't have a spelling reform is the fact that the are so many accents that no one would be happy and it would take a lot to change people. Besides, I like the fact that you can see some etymology is English words a lot more than some other languages that've had spelling reforms.
No such luck as 'knowing' unfortunately! You just have to learn every word with its spelling. A few rules become clear after a while, e.g. verbs in -ω but never in -ο, feminine words ending in -η but not in -ι (that's neutral). In general though Greek spelling is something you learn and accept as is. :)
Like the ch in "loch" or "Chanukah" which is also spelled "Hannukah". If you take the k and try to roll the sound, like you would roll an r, only you are making a heavy, breathy sound. This is a fricative sound that is unvoiced and produced in the velar area. http://www.internationalphoneticalphabet.org/ipa-sounds/ipa-chart-with-sounds/
It's funny you'd mention Hannukah here, in the Greek section of DuoLingo. The thing is, Hannukah in Hebrew means celebration of a new place (or however is correct to put it in English), and the holiday Hannukah refers to Hanukat Beit ha-Mikdash [ha-Sheni], celebration of the [Second] Temple. And it's not the primary opening of the Temple that is celebrated, but its liberation from the forces of Greek king Antiochus, whose affairs in Canaan were mainly devoted to prosecution of Hebrews. Later on, Greek forces razed the Second Temple down anyway. So cool Greeks today are πολύ καλό, and I love Greece and its folks, unconditionally and regardless ^_^
Yes, fairly similar: both have two pronunciations depending on the surrounding sounds, and the two pronunciations are similar as well (though the "back" sound in German is typically a bit further back than the Greek one, I think).
The main difference, perhaps, is that the choice of sound in German depends on the preceding vowel or consonant; in Greek on the following vowel or consonant.
Because most of human history people have cared more about the ancient version of the language. In Ancient Greek χ was a type of k-sound, while φ was a type of p-sound, like θ is a type of t-sound to put it simply. Using more exact terminology: χ, φ and θ were all aspirated stops.
I can not wait to study greek I was on holiday and picked up good morning, good night and good afternoon I have never forgoten than and I have never forgoten my greek friend from years ago like little Mariana and feel away from the ability to read greek at a young age, I love the music I think of going back anytime I miss it.