Hello
How to find out the subject in the sentence--Look for what is being talked about in the sentence and then try to catagorize it into the catagory of the subjects like-first,second,third and so on.
Example-- The class is seated
In the above example,it is being talked about "class".Now class will come under which catagory of subject, i.e. It will come under the third catagory of subject. As in the sentence it is being talked about a third entity because the first person is speaking the sentence,the second person is listening the first person.