This construction is the same in form as that shown in Lesson 18-1, but the predicate expresses feelings about one's own body (part). It is also used to talk about one's health conditions. Unlike Structure 1 above, this cannot be paraphrased by the "N1の N2は ～ " construction. In other words, わたしは 頭が いたいです(I have a headache) CANNOT be changed into わたしの 頭は いたいです.
When the subject is in third person, a commonly used alternative is Aさんは Nが ～と 言っています(Mr./Ms. A says that s/he ～).
わたしは 頭が いたいです。
I have a headache.
わたしは 手足が だるいです。
My arms and legs feel heavy/languid.
わたしは 気持ちが いいです。
I feel good.