In life, there are things that we must or must not do whether it's taking out the trash or doing our homework. We will cover how to say this in Japanese because it is a useful expression and it also ties in well with the previous section. We will also learn how to the say the expression, "You don't have to..." to finish off this section.
If you don't know what 「だめ」（駄目） means, I suggest looking it up in WWWJDIC and clicking on the [Ex] link to see the example sentences. It essentially means "no good" but carefully check out the example sentences because it can be used in many different ways. The other two key words in this section are 「いけない」 and 「ならない」 and they have essentially the same basic meaning as 「だめ」. However, while 「いけない」 can be used by itself, 「ならない」 must only be used in the grammar presented here. In addition, while 「いけない」 and 「ならない」 conjugate like i-adjectives they are not actual adjectives. Let's learn how to use these words to express things that must not be done.
The difference between 「だめ」、「いけない」、and 「ならない」 is that, first of all, 「だめ」 is casual. While 「いけない」 and 「ならない」 are basically identical, 「ならない」 is generally more for things that apply to more than one person like rules and policies.
You may have predicted that the opposite of "You must not do" would use 「いける」 or 「なる」 because they look like the positive version of 「いけない」 and 「ならない」. However, 「いけない」 and 「ならない」 must always be negative, so this is not correct. In actuality, we still use the same 「だめ／いけない／ならない」 and use the opposite of the verb that goes in front of it instead. This double negative can be kind of confusing at first but you will get used to it with practice. There are three ways to conjugate the verb before adding 「だめ／いけない／ならない」 and two of them involve conditionals so aren't you glad that you just learned conditionals in the previous section?
The first method is the same as the "must not do" grammar form except that we simply negated the verb.
The second method uses the natural conditional that we learned in the last lesson. Literally, it means if you don't do something, then it automatically leads to the fact that it is no good. (In other words, you must do it.) However, people tend to use it for situations beyond the natural consequence characterization that we learned from the last section because it's shorter and easier to use than the other two types of grammar.
The third method is similar to the second except that it uses a different type of conditional as explained in the last lesson. With the 「ば」 conditional, it can be used for a wider range of situations. Note that since the verb is always negative, for the 「ば」 conditional, we will always be removing the last 「い」 and adding 「ければ」.
It may seem like I just breezed through a whole lot of material because there are three grammar forms and 「だめ／いけない／ならない」 adding up to nine possible combinations (3x3). However, some combinations are more common than others but I did not explicitly point out which were more common because any combination is technically correct and going over style would merely confuse at this point. Also, keep in mind that there is nothing essentially new in terms of conjugation rules. We already covered conditionals in the last lesson and adding the wa particle to the te-form in the beginning of this section.
Although we spent the last section explaining 「～なければ」 and 「～なくては」, the reality is that because they are so long, they are practically never used in real conversations. While they are often used in a written context, in actual speech, people usually use the 「と」 conditional or the various shortcuts described below. In casual speech, the 「と」 conditional is the most prevalent type of conditional. Though I explained in depth the meaning associated with the 「と」 conditional, you have to take it with a grain of salt here because people are inherently lazy.
You may have been grumbling and complaining about how long most of the expressions are just to say you must do something. You can end up with up to eight additional syllables just to say "I have to..."!
Well, others have thought the same before and people usually use short abbreviated versions of 「なくては」 and 「なければ」 in casual speech. Teachers are often reluctant to teach these overly familiar expressions because they are so much easier to use which is bad for times when they might not be appropriate. But, on the other hand, if you don't learn casual expressions, it makes it difficult to understand your friends (or would-be friends if you only knew how to speak less stiffly!). So here they are but take care to properly practice the longer forms so that you will be able to use them for the appropriate occasions.
Right now, you may be saying, "What the?" because the "abbreviations" are about the same length as what we've already covered. The secret here is that, unlike the expressions we learned so far, you can just leave the 「だめ／いけない／ならない」 part out altogether!
The 「と」 conditional is also used by itself to imply 「だめ／いけない／ならない」.
There is another 「ちゃ」 abbreviation for things that you must not do. However, in this case, you cannot leave out 「だめ／いけない／ならない」. Since this is a casual abbreviation, 「だめ」 is used in most cases.
One very important difference for this casual form is that verbs that end in 「む」、「ぶ」、「ぬ」 use 「じゃ」 instead of 「ちゃ」. Essentially, all the verbs that end in 「んだ」 for past tense fall in this category.
On a final note, in general, 「ちゃ」 sounds a bit cutesy or girly. You've already seen an example of this with the 「ちゃん」 suffix. Similarly, 「なくちゃ」 also sounds a bit cutesy or childish.
Now let's learn how to say either that it's ok to do or not do something. I decided to shove this section in here because in Japanese, this is essential how to say that you don't have to something (by saying it's ok to not do it). The grammar itself is also relatively easy to pick up and makes for a short section.
By simply using the te-form and the 「も」 particle, you are essentially saying, "even if you do X..." Common words that come after this include 「いい」, 「大丈夫」, or 「構わない」. Some examples will come in handy.
In casual speech, 「～てもいい」 sometimes get shortened to just 「～ていい」 （or 「～でいい」 instead of 「～でもいい」 ）.