As is the case with most languages, there are so many types of slangs and abbreviations in Japanese that there is no way to categorize them in a unified manner. Trying to learn slang by memorizing rules is probably close to impossible because of the inumerous number of inconsistencies.
Here, I’m just going to go over a couple of common types of slang so that you can get an idea of how it works. Like I mentioned previously, it is impossible to fully and comprehensively explain these types of things but it can still be useful to get familiar with the general idea.
Fortunately, slang is very easy to pick up by speaking and listening because they naturally come about from people finding easier ways to say something.
In short, the driving force behind Japanese slang is to make things easier to say. There are two cardinal rules that go along with this idea.
1) Make it shorter.
2) Be lazy.
Since 「ん」 is the only letter that lacks a syllable, it is the shortest sound in the Japanese language. As a result, it is often used to substitute for other longer letters that require more energy to pronounce; in particular the tongue rolling 「ら、り、る、れ、ろ」 sounds.
One of the most common example of this is the substition of 「ら」 in 「わからない」.
– [Do you] know where Misa-chan went?
In fact, you can do the same type of substitution for any 「～らない」 negative verbs.
– Don’t really know but everybody said it was really great.
– Doesn’t [your] head become hurting when you read a book in dark room for a long time?
Another common substitution is the 「いる」 from the 「～ている」 enduring state form.
This one’s a bit tricky because you can’t actually end a sentence with just 「ん」, you always need something to come after it.
– [We need something to come after 見てん]
-Hm? Well, [I’m] watching movie now, but?
「ん」 is sometimes even substituted for letters in regular words such as 「つまらない」.
– It’s boring here so let’s go somewhere.
There are many more examples of 「ん」 substitution for abbreviations. One of the most common examples is the subsitution for 「のだ」 as seen here. Another example is the 「ん」 substitution for the 「ない」 in negative verb conjugations as seen here. If you spend quite a large amount of time speaking Japanese, you might find yourself making these substitutions yourself unconsciously.