"I have studied" means "I have studied Italian at some time in the past". Maybe last year or 10 years ago. "I have been studying" means I have been doing it in the recent past and I'm probably still doing it. "I have been studying Italian until last term/until yesterday"

In a few cases, there is little or no difference between the two forms. For example, there is no significant difference between 'How long have you lived here? and 'How long have you been living here?' or between 'How long have you worked here? and 'How long have you been working 'here?' However, in most situations, there is a difference. The continuous form often focuses on the action and its duration, whereas the simple form focuses on the result. Here are some examples: I've been reading a lot lately. / I've read six novels this week. He's been driving for hours / He's driven 200 miles. She's been dancing all evening / She's danced with every boy in the room

"I have studied" is Present Perfect, which means that the event happened in the past. There is no specific time element required, although we can add a duration like "for five years". Remember that the preposition "for" is important to express duration. "I have been studying" is Present Perfect Progressive, which means that the action happened at a specific time in the past and continues to the present. You can add a duration phrase here, too, such as "for five years" or "since 2009". Again, the prepositions "for" and "since" are necessary. :)