Learning how to use structure "If clauses with modal" at English class

This structure is not easy and not difficult. It is easy because I have seen and done it much. It is difficult because I haven't done it for a long time. But I remember little by little through exercises. I have to done hard to use clearly.

Sister Hong's birthday

Today is not sister Hong's birthday. Her birthday is yesterday. Today, she organizes a birthday party. She invites members of Xuan Sung group which she is a team leader through Facebook group. Many members read lately or don't read so they don't come. Although only it has 10 people, it is very interesting. We drank tea-milk, then eat stir-fry corn and finally, we cut gateau and eat together. While eating, we sang and chatted. About 2 pm, the party ended.