Iversen wrote:One simple criterion is how well I remember new words.
I have a theory that recognising words is a bit like recognising faces, so your brain is better at distinguishing between phonemes that your brain was exposed to as a child during development, and it takes longer to learn to distinguish between foreign phonemes as an adult.
When I was learning the Korean language, I had very little difficulty with the writing system (as anyone who's learnt Hangul can attest to), but I had GREAT difficulty trying to remember vocabulary. Eventually, I figured out what the difficulty was. Certain phonemes that are extremely high frequency in Korean such as anything with the "J" sound (ㅈ) are extremely rare in English (see https://en.wikipedia.org/wiki/Letter_frequency). So as I was learning new vocabulary as a beginner, what I would do for each word was to think of an English word that sounded similar and then make a visual and auditory mnemonic in my brain to relate the two. And if it didn't sound anything like any English word, and this was typically when a word had a "J" sound and none of the other consonants in the word were high frequency in English, what my brain would do is just remember it as "That's the word that has the J sound". Because I had nothing else easy to anchor it to in my brain, that was my way to distinguish the word from other words: i.e. This word is different because it has a "J"! This worked at first as a beginner when I hadn't yet seen many words, but then as I kept trying to learn more and more words, I got really confused whenever I encountered *another* word with a "J" in it because try as a might, my English brain couldn't find any other distinguishing characteristic in the word to remember it by. Basically, all of the words with "J"s in them "sounded the same". A lot of the time I was saved because some of those words that have "J"s in them also have other consonants in them that I can anchor to English-sounding words and remember them that way, but there were a whole bunch of words without anything to anchor them to that I just couldn't easily remember, and they all just sounded the same. Another factor with Korean is that it has quite a few sounds that just don't occur in English at all.
By comparison, I'm finding Japanese (despite it's much harder writing system) to be much easier to learn because its vocabulary has got lots of T's, S's and other consonants that are also really common in English, it lacks all of the sounds that are infrequent in English, and virtually all of the Japanese phonemes exist in English. When I began learning Japanese vocabulary I was blown away by how easy it was, and interestingly it didn't get any harder as I got up to the more advanced vocabulary.