It is often said that textbook Japanese and real Japanese are different. Easy phrases that learners come across in textbooks like 食べてしまいましたか hardly prepare learners for real-life equivalents like 食べちゃったん？
A big part of this difficulty is because of joining. Much like English, the Japanese language often joins words that begin with vowels to the previous sounds.
Japanese vowel joining.
One of the first uses of this that learners encounter is the N4 grammar form ～ておく (~ in preparation for a later act). To understand how it joins, it is worthwhile recording yourself saying something like 置 quickly. What you should notice is that the お and て begin to blend together the quicker you say it. Unsurprisingly, Japanese people often combine these in speech; you will often hear 置いておく pronounced as 置いとく in casual speech.
A similar thing can be heard in words that include the word あげる (to give), which, of course, also begins with a vowel. Here again the て and the vowel combine, so for example してあげる is often changed to したげる when said quickly.
So what about my earlier example? Where exactly does “食べちゃったん？” come from? The basic situation is that してしまう becomes しちゃう and 読んでしまう becomes よんじゃう, respectively when said in usual speech.
While this may seem like a big change, there is a good reason for this. If you say, for example, してしまった quickly, you should notice that it is pretty tricky to say. This is why it is often simply reduced it to しちゃった, as it makes it much easier to say quickly.
This general rule will help with these grammar forms, as a lot of learners have difficulty working out whether a verb should be ちゃう or じゃう, for example. The simple rule is to try saying the right and wrong thing and seeing which is easier to say. Try saying the grammatically wrong しじゃう or よんちゃう quickly and you will soon see that both are real tongue twisters compared to the grammatically correct しちゃう (I did it) and よんじゃう (I read it).
Other common examples
There are many other common examples of these blends, but here are some of the most common ones:
The する-verb in しなければ いけない (I must ~). Once again, the part that is trickiest to say is contracted to しなくちゃ いけない.
In spoken language, なければならない (I have to ~) is sometimes replaced by なきゃならない. Similarly, なければいけない (I must ~) is often changed into なきゃいけない.
Some of these grammar forms have become so common that it is difficult to even remember the original form. A good example is だろう, which is actually from the form であろう. These days, it is very rare to hear anything other than だろう said and many people don’t even realize it is technically a blend.
As most Japanese people use かもしれない (Possibly) a lot, it is probably not surprising that it is often contracted. Interestingly it is often reduced to かもしんない although there are regional variations. Of course, many people simplify it further to just かも.
While they may seem tricky at first, these are all things that when mastered will help you to speak Japanese more naturally and start the transition to Japanese beyond the textbooks. As young people say 勉強しちゃっただろうか (Shall we study?).