What is the real difference between impiegare, necessitare (di qualcuno/qualcosa) and aver bisogno di?
I know necesitar from Spanish, and avoir besoin de... from French, but Italian having both and impiegare is new to me.
Can please someone enlighten me about which one to use, and when? Are they interchangeable?
Impiegare is often used with time notions.
For exemple: To take me 1 hour to go to school. Impiegare 1 ora per andare a scuola.
Necessitare is mostly written. By oral, you'll nearby never heard it. The only way you can use it is in manuals, in recipes, lists of material you need... It's not very often used.
For exemple: To cook this cake, you need 3 eggs. Per cucinare questa torta, necessitano 3 uova.
Aver bisogno is often used with objects or people. It means more or less "to need" in English. Aver bisogno is very often used by oral.
For exemple: I need your help. Ho bisogno del tuo aiuto.
The most general Italian verbs/phrases to convey the sense of “to need” are avere bisogno (di) and occorrere. For instance, “I need help” is Ho bisogno di aiuto or Mi occorre aiuto. The latter is less colloquial, more literary than the former.
Another way to say this is with the impersonal use of the verb servire; so, the above example could also be translated as Mi serve aiuto. In a sense, avere bisogno emphasises the actual need, while servire borders on “I could use [whatever]”.
Some uses of “to need” are better conveyed by a colloquial construction with the verb volere. To say that to make some cake one needs two eggs, you might say Per quel dolce ci vogliono [but also servono] tre uova. Or ci vuole un bel fegato for “it takes a lot of guts [to do something]”.
Necessitare is more along the lines of “to be in need of”, but it sounds presently quite affected. Note that in modern Italian (and in this meaning) it is usually constructed with di: so, for the previous example, Necessito di aiuto. Essere necessario is somewhat more usual, especially in regulations and the like: È necessario inserire la password (“You have to insert your password”).
Finally, the main meanings of impiegare are parallel to those of “to employ”, both in the “give work” and in the “make use of” senses. It may be used in a vaguely similar sense to the “need” meaning, in sentences like Il treno impiega tre ore per arrivare a Firenze, that is, “It takes three hours for the train to reach Florence” (but most people, in ordinary speech, would probably say Il treno ci mette tre ore).
For further nuances about some of these terms, the Dizionario dei sinonimi by Niccolò Tommaseo is as always enlightening, even if it is about 150 years old. For the present topic, who desires to strike out into 19th-century Italian will be rewarded by reading here.
Impiegare means that you use something for a purpose. For instance, you can use impiegare to say that you use your bike in order to go to school.
Necessitare means that you really need something or someone, which is extremely important for your purpose.
Aver bisogno di means that you need something or someone, but it's not really necessary, you simply would like to have it.
Sorry I can't explain better than this ;)
