In the Japanese language politeness levels play an important role in defining status, position, "direction", and intimacy. Not using the correct level of politeness can be confusing, unprofessional, or even insulting to the Japanese.

Informal language
The informal language should only be used among friends and family. It creates an atmosphere of intimacy by which people who don't know you would feel very uncomfortable. Using the informal construction when speaking to whom you should be showing respect, like a teacher or a boss, is considered insulting. All sentences that lack polite forms and verbs are considered to be informal.

The basic polite forms: ます (masu) and です (desu)
Using the basic polite forms puts some distance between you and the person you are speaking to. At the same time it shows respect to the person you are speaking to, without being humble. This basic form can be constructed by adding ます (masu) to the ren'youkei conjugation of verbs, or です (desu) to the rentaikei conjugation of verbal adjectives. です (desu) by itself is the polite form of だ (da).

Instead of ます (masu) you can also add のです (no desu) to the rentaikei conjugation of verbs. This construction can be used to put some variety into your sentences, or to make a sentence polite if you forgot to use the ます form.

Informal		Polite
買う	kau	買います	kaimasu
		買うのです	kau no desu
書く	kaku	書きます	kakimasu
		書くのです	kaku no desu
出す	dasu	出します	dashimasu
		出すのです	dasu no desu
待つ	matsu	待ちます	machimasu
		待つのです	matsu no desu
食べる	taberu	食べます	tabemasu
		食べるのです	taberu no desu
見る	miru	見ます	mimasu
		見るのです	miru no desu
する	suru	します	shimasu
		するのです	suru no desu
来る	kuru	来ます	kimasu
		来るのです	kuru no desu
高い	takai	高いです	takai desu
		高いのです	takai no desu
難しい	muzukashii	難しいです	muzukashii desu
		難しいのです	muzukashii no desu
高い	takai	高いです	takai desu
		高いのです	takai no desu
難しい	muzukashii	難しいです	muzukashii desu
		難しいのです	muzukashii no desu

When speaking with friends this form should be avoided as it puts distance between you and your friends, and can give the impression that they did something wrong. On the other hand, if you have offended your friends in some way, using this form may restore some of the respect your friends had for you.

NOTE: When conjugating the polite forms with です (desu) the main verb is conjugated. です (desu) remains unconjugated. The only exception is when です (desu) is used by itself as the polite form of だ (da). In that case of course です (desu) is conjugated since then that's the main verb. When conjugating the polite verbs with ます (masu) most of the time ます (masu) is conjugated.

Polite verbs and words
Japanese has a number of polite synonyms. When using these you upgrade the overall politeness level without the use of humble and honourific words.

You can make any noun polite by adding the honourific お (o) or ご (go). お (o) should be used with kun'yomi words, and ご (go) with on'yomi words (see lesson 3).

You can also use the passive form to increase the politeness. The polite passive form maintains the normal construction a normal sentence would.

Humble and honourific verbs
In some cases the normal polite forms may not do the trick. Honourific and humble verbs are used when you need to show more respect than normal. These circumstances include: dealing with customers, asking favours, and making up for errors and mistakes. Also when speaking to people who are much higher on the social ladder it may be appropriate to use these verbs.

Humble verbs can only be used for yourself and honourific verbs only for other people. Using honourific verbs for yourself, even as a joke, won't be understood by Japanese people as they will automatically assume you are talking about them.

You can make any verb honourific by using the following construction:

お + ren'youkei + になる (o-ren'youkei ni naru)

Related pages:
The ren'youkei
Mizenkei +(ら)れる ((ra)reru)
Polite forms
Passive forms
To be or not to be