In the modern Arab World, Fusha is the language of all writing, official media, news broadcasts, politics, stuff like that. It's basically a slightly simplified version of Classical/Qur'anic Arabic with new words and expressions for modern life. The grammar and vocabulary for the most part is identical, and Arabs call both MSA and Classical Arabic Fusha.
3ammiyyah refers to the dialects that people speak to each other with. In the Maghreb they call it Darija instead. 3ammiyyah is different from country to country and town to town. When people talk to each other and on less official media like dramas or talk shows they use 3ammiyyah. Here in Jordan they use Jordanian Arabic/3ammiyyeh urduniyyeh, which is quite similar to Lebanese, Palestinian, and Syrian Arabic.
Some people have compared this situation with Late Medieval Europe and into the Early Renaissance. Then, Latin was the language of the Church, and all theological and scientific knowledge. Dialects across the former Roman Empire like Old French, Old Spanish, etc. were considered vernaculars and less prestigious than Latin, but no one actually spoke Latin in their daily life except for monks and giant nerds. Later, French, Spanish, Italian, etc. gained legitimacy and are now considered their own languages. Some people say Arabic is like that. Fusha is like Latin, highly prestigious and official but no one actually speaks it day to day. The various dialects are different enough to be their own languages. But we still call the entire diglossia one language: Arabic.
So as a student living here in Jordan, I learn Fusha in class, just like how I learned Fusha back home. But when I go into the city, I'm surrounded by people speaking 3ammiyyah. Unlike Arabs I speak Fusha better than 3ammiyyah and feel more comfortable speaking Fusha. Also I find it harder listening to 3ammiyyah than the Fusha that I'm used to when people are speaking back to me. So the diglossia adds another layer of challenges when learning Arabic.
But when I do speak with Arabs and use Fusha, I sound excessively formal or antiquated, like a guy on the Streets of Boston speaking King James Bible English. It puts a slight barrier between me and the other speaker, on top of the language barrier that's already there. It's harder to have a natural conversation with Mr. KJB English than with Mr. 21st Century English.
One time when having a conversation with a taxi driver, I couldn't understand the question he was asking after he repeated it a couple times. And knowing that I was a student taking Arabic classes he asked "should I say it in Fusha?" Knowing that honestly Fusha was easier for me.
It's made me more aware of the connotations that certain Fusha or 3ammiyyah words have when talking to people. Saying "kaifa al-Haluk" instead of "keefak" for "how are you" for example, connotes more formality than is appropriate for a cab ride. So when speaking to people outside my class I try to use as much 3ammiyyah words as I can and conjugate verbs in an 3ammiyyah way. But because Fusha's just easier for me I end up speaking this inconsistent sorta-3ammiyyah sorta-Fusha mix. And when I return to class or my language partner I have to remember to switch 3ammiyyah words back with Fusha words.
Obviously every language has formal and informal registers. But Arabic takes it to a slightly greater level, with 3ammiyyah having notable differences in grammar and differences in pronunciation. A sentence like "I want to go down to Rainbow Street" would be:
Fusha: "ureedu an adhhaba ila ash-shaari3 Rainbow" أريد أن أذهب ألى الشارع رينبو
3ammiyyah: "biddi ruh 3ash-shaari3 Rainbow" بدي اروح عالشارع رينبو
A bit more different example would be like "what do you want me to say", which would be:
Fusha: "madha tureedu an aquula?" مادا تريد أن اقول؟
3ammiyyah: "shu biddak eHki?" شو بدك احكي؟
A bit more different example would be like "what do you want me to say", which would be:
Fusha: "madha tureedu an aquula?" مادا تريد أن اقول؟
3ammiyyah: "shu biddak eHki?" شو بدك احكي؟
Also some sounds shift from Fusha to 3ammiyyah. For example, many grammatically feminine words have an "aah" ending in Fusha that becomes an "eh" in 3ammiyyah", or maybe an "é" depending on where exactly you're from. Also some consonants change, like the ق/q sounds like "g" as in great, or ظ/DH sounds like "z" or ث/th sounds like "t". So a word like qahwah/قهوة in Fusha sounds like "gahweh" in 3ammiyyeh.
So when I enter a cab or walk into a shop I think to myself "remember to switch out the Fusha words, add the b- prefix to verbs, don't use Fusha particles/modals, take out the vowels, don't speak so clearly, merge your consonants, etc.". When I go back to class I think "remember, stop saying "بس" and "كتير" and "شكله", conjugate words in Fusha, put the particles/modals back, pronounce all the vowels, remember the case endings, etc."
So when I enter a cab or walk into a shop I think to myself "remember to switch out the Fusha words, add the b- prefix to verbs, don't use Fusha particles/modals, take out the vowels, don't speak so clearly, merge your consonants, etc.". When I go back to class I think "remember, stop saying "بس" and "كتير" and "شكله", conjugate words in Fusha, put the particles/modals back, pronounce all the vowels, remember the case endings, etc."
I suppose it's kind of like the difference between Black English and Standard American English. There's grammatical features, like different tenses and syntax, that don't exist in SAE that are in BE and vice versa. Also BE has different sounds and phonology than SAE. They're still considered varieties of the same language, but BE has much less prestige. Of course, people speak SAE in their day-to-day lives and it's not a perfect analogy.
So it's another layer of challenge when learning Arabic, but it's also kind of a cool phenomena that doesn't really have a parallel in non-diglossic areas like back home in the States.
