What does the word 'field' mean in this context?
I'm not sure if this is the case here, but in some theories of linguistics, 'field' is a technical term that refers to the way a text represents reality (or our experience of reality). I don't know what 'inner' and 'outer' fields would be, unless they could be referring to 'within the clause' and 'outside the clause'. If this is the case, the general idea is 'context', i.e. you can work out the tense from the context (e.g. 明天，昨天）

Who is the original author of the text? Knowing their background could be helpful in understanding what framework they were using.

Okay, based on further info it looks like this answer is completely off-base.