I think it belongs in syntax... not too sure about that though. It seems to incorporate both aspects of lexis (e.g. past participle forms) as well as syntax (verb phrases). If I were you, I'd first see if it was important enough to comment on (e.g. if it plays a major role in supporting some function/purpose) and then maybe I'd use a different approach to analysis, if I could think up 3 points encompassing the text on which to build 3 paragraphs (you don't need to structure your AC by subsystem!).
For example, I could mention tense in a paragraph on the informative function of some text, claiming that the tense reflects the temporal relevance of the ideas expressed.
Other than that, personally I wouldn't group it into syntax paragraphs