In general, I hate the idea of lists or any form of measure or statistic when it comes to fluency. I believe that being fluent means being able to communicate on common subjects without a translator or dictionary, not some numerical measure of how many words you know or how many lessons you've completed. Remember, it's not a contest.
Nevertheless, there are a few lists available that will be extremely beneficial to you regardless of where you are in the learning process. Of course I'm referring to frequency lists.
What the heck is a frequency list?
A frequency list is simply a list of the most commonly used words in a language. Linguists compile books, transcripts, subtitles from television and movies, etc., and feed them into a computer, and the the computer reports how many times each word appeared in the sample data.
In general, more samples means more accurate data. But often it's more interesting to choose more targeted data. Wiktionary, for instance, tends to prefer movie and television subtitles over literature, probably on the assumption that it is more representative of actual use of a language.
Frequency lists are usually sorted by popularity, with the most commonly used words appearing first. Sometimes, if you can get your hands on a well-prepared frequency list, it will also provide lemma data — that is, word usage, part of speech, infinitive form, etc.
They're generally easy to find on Google. Just search for "frequency list [language]", where [language] is the language you are studying. For instance, frequency list italian.
How to use a frequency list
The beautiful thing about having a frequency list is that you can always start at the top with the full confidence that the next word you learn is the next most commonly used word. It's like someone has done the work of finding out what the most important words are and handed them to you on a silver platter!
Just print out your list, and then periodically — maybe once every few days, or once each week — start at the top and mark out the words you know. You should be able to describe the word's meaning in English (or whatever your native language is), and you should also feel confident that you would know that word if you needed it in a conversation. Keep in mind that it's not a race — crossing out a word you don't know is only cheating yourself later on.
Take breaks, do it one page at a time, etc. (Space it out; don't make it hard on yourself.) Your first time through, you'll cross out a lot. After that, it'll just be the new words you've learned that week.
It's usually pretty easy to find the top 1000 words, and without a doubt you should know all of them. After your first pass, focus your attention on learning the most commonly used words on the list which you don't already know. Look them up on WordReference, try them out in conversation, and make yourself comfortable with their use.
If you look a bit harder, you can usually find more, like the top 5000 words. It's nice to set a high goals, and I certainly encourage you to try to learn all 5000 in one year, but that's a very aggressive goal.
Last year I tried to learn the top 5000 words in Russian and ended up around 3300. One might consider it a huge failure, but that means understanding almost 80% of word forms in text. That's good enough to understand most of what's printed in the news, and almost all of what is said in a typical chat with a friend.
If you're studying Italian, I have already done the work of formatting the top 5000 words into a convenient, four-column, Italian frequency list so it should be compatible with Mac, Window, Linux, iPad, Kindle, iPhone, or just send it to a printer and carry the pages with you.
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