How difficult is it for to learn German?
It depends on your native language. According to the Foreign Service Institute, they estimate that German takes around 30 weeks to learn (or 750 hours) for English natives. (Source) Personally, I find it easy in some aspects, but difficult in others. One of the easy parts is vocabulary, while one of the difficult parts is grammar.
For the FSI stat, yes. I formatted it so that if you click on it, you will be sent to the page with the FSI language difficulty rankings.
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It indeed depends on your Native Language. I found vocabulary to be very easy as it is very similar to English, but grammar very difficult as German retains cases and can sometimes have a different word order.
I would say it's harder than the other common romance/germanic languages of Europe (besides Faroese or Icelandic), but easier than the slavic languages of eastern europe.
I'm finding it more enjoyable to learn than French. Although like French it's annoying how you have to remember what's male and female. Many words are similar so I can remember. Such as man is mann, boy is junge, sounds like young. Girl is madchen, sounds like maiden. See you later is bis spater
The grammar is cancerous, but the vocab is easy, so people would still understand you. I think it really depends on your native language and how much you speak/how much exposure you get every day. I know people who have been living here for 2 years & speak it perfectly, some for 3 years and can't say very much with terrible grammar, hell, there's even someone who has been living here for 20 YEARS and still only speaks russian.
It's all about motivation & discipline~~ If you just learn it for fun I don't think you'll come very far.