Great question, Asahi! Here's the answer: I am going to play soccer. This sentence means that you plan to play soccer soon. It's a common way to express future intentions in English. I am to play soccer. This sentence is less common in everyday conversation. It implies a more formal or scheduled event, almost as if it's a duty or a part of an official plan. Even though both sentences can indicate a future action, "I am going to play soccer" is more natural and widely used in casual speech. The extra "going" helps convey the idea of an intended or planned action in the future, making it sound more conversational.

@Jack IELTS. I think it's country- specific. In fact, am certain of it. It is a Briticism, insofar as I've heard. Write or say that here and you will be corrected every time.

The second sentence is incorrect grammar. Only the first sentence is correct. 'Going to' expresses what a person or thing is about to do, or plans to do.

Both are grammatically perfect and have several possible meanings. It is impossible to say precisely what either one means without further context. #1 could mean any of the following, but the first two are the most common: "I am physically in motion towards a place where I will play soccer" "I will play soccer" (a pure statement about the future) "I intend to play soccer" "Playing soccer is part of my life" (a timeless statement about the past as well as future) #2 is more in need of context than #1, but the possibilities are wide and limited only by the imagination of the speaker. Here are some: "I affirm that I will play soccer" "It has been decided that I will play soccer" "I am a player in the (or 'a') soccer game" "My role (in some activity under discussion) is that of soccer player" The interpretations that I have given do not exhaust the possibilities. Present participles, especially, do not have precise meanings. That is why you cannot pin down what they say. Even the part of speech can be ambiguous: a present participle can be a noun, adjective, or adverb, depending how it is used. It simply repackages a verb as something else that only must be somehow related to the verb. This flexibility gives enormous expressive power to the language. A skilled writer or speaker can let the imagination soar by using them cleverly. Imagine that a friend walks up to you while you are walking on the sidewalk and says "I am swimming". What does it mean? There is no water around. The possibilities are endless. This is typical with present participles.

to be going to -- you are planning to be to -- you are supposed to perform the action