"The child throws the ball."
Translation:아이가 공을 던집니다.
Korean doesn't distinguish between definite (the) and indefinite (a/an) like English does; -는 marks the conversation topic and -가/이 marks the sentence subject. Korean grammar is extremely different from what you see in Western European languages, but if you keep practicing you'll get a feel for it.
The particles 이/가 and 은/는 they refer to the topic/subject of the sentence (I don't know well the difference between those two). The particles 을/를 refer to the object of the sentence.
Think like this: the verb is "to throw", then you identify who (or what) is performing the action (subject). In this case it is 아이 (the child). Then identify what (or whom) is receiving the action (object), what is being thrown. In this case it is 공 (the ball).
Of course, not every verb needs to carry an object.
I hope it helps.
Misread your question. Probably because of the speech level, you should flag it.