やる is rough and colloquial version of する。 You can use it in very casual conversation between friends, but never in formal speech or writing. Although they are inter-changeable in many cases, there are some differences. -When the "do" expresses some mental, abstract or conceptual actibity, やる doesn't fit well. -やる doesn't fit if the subject of the verb is a thing with no will, not a human or animal that have a will. -やる always comes with particle を. It can't make a compound verb as する can. (In casual speaking, particle を is often dropped, but it's another issue.) In examples below, "○ " means "it's ok", and "×" means "not ok." 野球をする→ ○野球をやる play baseball 宿題をする→○宿題をやる do homework したいことをする→○やりたいことをやる do what you want to do 結婚式をする→○結婚式をやる have a wedding ceremony 愛する →　×愛やる love 入学する→　×入学やる　enter the school 結婚する　→×結婚やる　get married 合格する　→×合格やる　pass the test 地球が自転をする→　×地球が自転をやる The earth rotates. In short, やる suits for actual physical action by a person or an animal. If you are not sure, just use する; that's safer and sounds politer ;) ---For further study---- Both する and やる have many more meanings other than simple "do." In those cases, you cannot replace one by another. ペットにえさを "やる"→ ×ペットにえさをする　"give" the pet some food 前畑選手、やりました！→×前畑選手、しました！Ms. Maehata "accomplish"ed ! ネクタイを"する"→ ×ネクタイをやる　"wear" a necktie 今日のランチはカレーに "する" →×今日のランチはカレーにやる　"choose/decide to have" curry rice for today's lunch 部屋を暗く "する"　→ ×部屋を暗くやる "make" the room dark

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