1. The students are playing soccer against the teachers.
(What is the part of speech of 'against the teachers'? Is it an adverbial phrase or an adjective phrase? Does 'against the teachers' modify 'soccer'?)
2. What's your favorite place at school?
- It's playing field.
- It's a playing field.
- It's the playing field.
(Which answer is grammatical?)
1. The prepositional phrase has to be adverbial since it's modifying "are playing" (verb).
2. Either the second or third answer is correct. You'd use the second answer if there are two or more playing fields, and your favorite place is one of them. You'd use the third answer if there's one playing field or if there are several and you are speaking generally (that is, you're happy on any playing field!).