we can use أحتاج in slang and fusha but أريد is only in fusha >>in slang u can say 3wez.in egyptian abgha in saudi..baddi in lebanon or jordian. when you need somthing you use أحتاج when you want somthing you use أريد for example: I need some money أحتاج إلى بعض المال I need some rest أحتاج الى الراحة i want to sleep أريج أن أنام i want you come with me أريدك أن تأتي معي

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they are both fusha the difference is want=أريد / ored from the verb want we have the noun will = إرادة= erada ex: I want to learn Arabic . oredo an ata3alama l3arabiah. أريد أن أتعلم العربية from the verb want we have the noun will = إرادة= erada need= أحتاج\ a7taj ex; I need money to buy a new car . a7tajo almala le ashtri syaratan jadedah. أحتاج المال لأشتري سيارةً جديدةً . from the verb need we have the noun need=الحاجة= al 7ajah . Ps : In my accent [ Syrian ] both need and want pronounced as [ bdde] بّدي