This is a very interesting question and at the same time too difficult to answer unless MP attempt a serious investigation using comparable data from different languages.

Thank you Mrs Nasseri,for your attention.The most representing abnormality in agrammatic aphasic is applying past tense instead of present and Future .Past tense is used for most of tenses in their speech even narration on pictures.Why?what is neurolinguistic reasons for that deficit?or It is morpho-syntactic (inflectional) processing in Phonetic forms ?...?????

I do not think so