This may help you: http://www.howtostudykorean.com/unit-3-intermediate-korean-grammar/unit-3-lessons-51-58/lesson-58/

U cant use 달 alone '달라고' is one grammar point. U can use 달라고 for reported speech. For example, 전화해 달라고 했다(same with above example), 그는 그가 만든 샌드위치를 여자친구에게 먹어 달라고 했다(he asked his girlfriend to eat sandwich that he made). I think this is very korean grammar so that little hard to translate. I recommand u to understand with many examples if you have any. I hope my answer can help u

To understand this you must first understand the concept of 라고. You can learn it here: http://www.talktomeinkorean.com/lessons/level-5-lesson-10/ Basically, you cannot use the verb "주다 (to give/do a favour)" to make the 라고 form. I.e, you can't say 주라고. Instead you have to change it to 달라고.