Mistakes in grammar can land you in the Hellfire
Ibn Salaah collects a chain of narration that al-Asma’ee said,
ان أخوف ما أخاف على طالب العلم اذا لم يعرف النحو أن يدخل في جملة قول النبي صلى الله عليه و سلم : من كذب علي معتمدا فليتبوأ مقعده من النار
لانه لم يكن يلحن فمهما رويت عنه حديثا و لحنت فيه كذبت عليه
“Indeed the most fearful thing I fear for the student of knowledge is to not have knowledge of grammar and [as a result] he enters into the statement of the Prophet [peace and blessings be upon him],
‘Whoever intentionally lies upon me then let him take his seat in the Hellfire.’ [Bukhari, Muslim]
[This applies to him] because the Prophet [peace and blessings be upon him] did not make grammatical mistakes. So whenever you narrate a hadith and you make a grammatical mistake [while narrating] then you have lied upon him.”
Source: Ibn Salaah’s introduction p. 400, Rawdatul ‘Uqalaa by Ibn Hibban p. 223