De で

Con il De/で si può esprimere:

• L'uso di un mezzo per fare qualcosa (scrivo "con" la matita 私は鉛筆で書く)
• L'uso di un mezzo/veicolo per andare da qualche parte (vado all'università "con" il treno 私は電車で大学へ行く)
• Lo stare in un luogo mentre si compie un'azione ("sto" studiando all'università 私は大学で勉強しています)
• Qualcosa fatto di un materiale specifico (quella casa è fatta "di" legno その家は、木でできています, dekite imasu/できています sarebbe "è fatta" )
• Per il prezzo (a 30 yen, 三十円で)
• Modo con cui si fa qualcosa utilizzando sostantivi (piangere ad alta voce 大声で泣く)
• Per indicare a che età (a 30 anni, 三十歳で)
• Per indicare il tempo necessario (in tre giorni 三日で)

(Fonte: me e Daniele)


De: indica il complemento di mezzo (Enpitsu de kakimasu, Scrivo a matita) e il complemento di stato in luogo con i verbi di azione (Daigaku de benkyō shimasu, studio all'università).


(Fonte: http://it.wikipedia.org/wiki/Lingua_giapponese#Particelle_di_caso)

The contextual 「で」 particle

The 「で」 particle will allow us to specify the context in which the action is performed. For example, if a person ate a fish, where did he eat it? If a person went to school, by what means did she go? With what will you eat the soup? All of these questions can be answered with the 「で」 particle. Here are some examples.

Examples

（１）　映画館で見た。
- Saw at movie theater.

（２）　バスで帰る。
- Go home by bus.

（３）　レストランで昼ご飯を食べた。
- Ate lunch at restaurant.

It may help to think of 「で」 as meaning "by way of". This way, the same meaning will kind of translate into what the sentence means. The examples will then read: "Saw by way of movie theater", "Go home by way of bus", and "Ate lunch by way of restaurant."

Using 「で」 with 「何」

The word for "what" （何） is quite annoying because while it's usually read as 「なに」, sometimes it is read as 「なん」 depending on how it's used. And since it's always written in Kanji, you can't tell which it is. I would suggest sticking with 「なに」 until someone corrects you for when it should be 「なん」. With the 「で」 particle, it is read as 「なに」 as well. (Hold the mouse cursor over the word to check the reading.)

（４）　何できた？
- Came by the way of what?

（５）　バスできた。
- Came by the way of bus.

Here's the confusing part. There is a colloquial version of the word "why" that is used much more often than the less colloquial version 「どうして」 or the more forceful 「なぜ」. It is also written as 「何で」 but it is read as 「なんで」. This is a completely separate word and has nothing to do with the 「で」 particle.

（１）　何できた？
- Why did you come?

（２）　暇だから。
- Because I am free (as in have nothing to do).

The 「から」 here meaning "because" is different from the 「から」 we just learned and will be covered later in the compound sentence section. Basically the point is that the two sentences, while written the same way, are read differently and mean completely different things. Don't worry. This causes less confusion than you think because 95% of the time, the latter is used rather than the former. And even when 「なにで」 is intended, the context will leave no mistake on which one is being used. Even in this short example snippet, you can tell which it is by looking at the answer to the question.

(Fonte: http://www.guidetojapanese.org/learn/grammar/verbparticles)

Using Japanese particle で (de) to show vehicle used

You can use particle で (de) to show what vehicle you are using for your movement using the following expression...

Vehicle で いきます/きます/かえります

Using some examples to show the usage of the above expression...

• わたしは じでんしゃ で かいしゃへいきます。
  (watashi wa jidensha de kaisha e ikimasu)
  Meaning: I go to office by bicycle.
  
• わたしはともだちと バス で がっこうへいきます。
  (watashi wa tomodachi to basu de gakkou e ikimasu)
  Meaning: I go to school by bus with my friend.
  
• わたしは でんしゃ で デパートへいきます。
  (watashi wa densha de depa-to e ikimasu)
  Meaning: I go to department store by train.

Using Japanese particle で (de) to show place for action

This function of で (de) is similar to "at" in English. When you perform an action at a place you can use で (de) to indicate the place. The expression is as follow...

Place で Action

Let's look at some examples...

• ちちはまいにち えき で しんぶんをかいます。
  (chichi wa mainichi eki de shinbun wo kaimasu)
  Meaning: My father buys newspaper at the station everyday.
  
• わたしはこんばん うち で テレビをみます。
  (watashi wa konban uchi de terebi wo mimasu)
  Meaning: I will watch television at home tonight.
  
• そふはまいあさロビーでたばこをすいます。
  (sofu wa maiasa robi- de tabako wo suimasu)
  Meaning: My grandfather smokes at the lobby every morning.

Using Japanese particle で (de) to show tool/mean for action

When you want to mention in a sentence what tool or mean is used in your action, you can use the particle で (de) as in the following expression...

Tool/Mean で Verb(Action)

Let's see some examples first on using tool in an action...

• わたしは えんぴつ で てがみをかきます。
  (watashi wa enpitsu de tegami wo kakimasu)
  Meaning: I write letter with the pencil.
  
• わたしは はし で ごはんをたべます。
  (watashi wa hashi de gohan wo tabemasu)
  Meaning: I eat the meal with the chopsticks.
  
• わたしは はさみ で かみをきります。
  (watashi wa hasami de kami wo kirimasu)
  Meaning: I cut the paper with the scissors.

Next let's see some examples on using mean in an action...

• わたしは にほんご で てがみをかきます。
  (watashi wa nihongo de tegami wo kakimasu)
  Meaning: I write letter with Japanese.
  
• わたしは えいご で さくぶんをかきます。
  (watashi wa eigo de sakubun wo kakimasu)
  Meaning: I write composition with English.

(Fonte: http://www.learn-japanese-adventure.com/japanese-particles-to-de.html)