The inability to form sentences by virtue of a brain disorder.(noun)
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Use "agrammatism" in a sentence
"_ -- Many children, long after they have overcome acataphasia and agrammatism, delight in inserting between words sounds, syllables, and words that do not belong there; e. g., they double the last syllable of every word and put an _eff_ to it: _ich-ich-eff_, _bin in-eff_, etc., or they make a kind of bleat between the words"
"Abnormal word order and a characteristic agrammatism are noted."