请帮我修改作文的语法错误

题目

请帮我修改作文的语法错误
Today was my father's birthday.I decided to give him a surprise,so I went to the shop and bought a gift.Then,I went home by subway train.Had been thing about how happy he was when he received the gift,I was so excited that I can hardly sat on my seat silently.At the next stop,got on a grandma,holding two big bags in her hands.She looked very tired.I began thinking to myself whether I should give my seat to her.Then about one minutes passed,I stood up and gave my seat to her."Thank you!"she said.When I heard these words,I felt very happy.I know I had done the right thing.Today was a great day!
这是一篇日记

答案

Today was my father's birthday.Today is my father's birthday.今天是你父亲的生日,这里没必要用过去时.so I went to the shop and bought a gift.后面可以加for him也可以不加.Had been thing about how happy he...