
Questions & Answers about kodomo ga haita.
が marks the subject of the sentence.
So in 子供が吐いた, 子供 is the thing/person that performed or experienced the action:
- 子供が = the child / the children as the subject
- 吐いた = vomited
A natural way to think about it is:
- It was the child who vomited
- The child vomited
In many basic sentences, が simply tells you who or what did the action.