刚 is a adv. 刚：it was not long before something happened. For example:他刚从北京回来。 刚才 is a noun. 刚才 means a period of time that passed just now. For example:刚才他在工作，这会儿开会去了。

Both of these words have the meaning"a moment ago",but they have difference : "刚”emphasis the time that someone did something ,for example:他刚出去（he went out just a moment ago.）and imply that he didn't come back . "刚才“ emphasis the fact that someone has done something,for example:他刚才出去了（he went out a moment ago),and i don't know if he is back or not.

刚bedeutet gerade。刚才bedeutet auch gerade,aber etwas getan oder passiert.manchmal sind die beide gleich.

good question. however, in fact, both of the answers are not exactly correct. 刚 or 刚刚 means "just". (in french, it means "venir de"+ verb). for example, u finished ur homework 10 seconds ago. then ur friend called u to ask u whether u had finished ur homework or not. then u can say "我刚做完"----"I just finish"----"je viens de finir (french)"-----------------why can we say like this? because u finished ur homework 10 seconds ago... it is really a very short time. another example, u r opening ur house-door and ur friend call u: "have u been at home?" u can say:"I just come back"----"我刚回来" 刚才 means "just now". this is easy. for example, 我刚才在买东西。----I bought something just now. for ur new question, u should say "我刚开始学中文" it means u just start learning chinese. this phrase has two meanings. 1) maybe u have been learning chinese for 10 years. and today, u just start to learn chinese. then u can say 我刚开始学中文---maybe u started to learn chinese 5 min ago. 2) u have been learning chinese for just one week. u know, one week for learning a new language is quite short. then u can say 我刚开始学中文 if u say 我刚才在学中文. it means exactly "I learned chinese just now"