Are you using the correct adjective declension? (Weak VS Strong)
Are you using the appropriate case for nouns, pronouns, and adjectives?
Are you using the appropriate case for verbs and prepositional phrases?
Are you using normal Old English phrasing, even if it is not the same as Modern English?
Is there an agreement in case, gender, and number between nouns, pronouns, and adjectives?
Are you not declining indeclinable words?
Are you using the partitive genitive for nouns affected by quantitive words (such as fela 'many, much')?
Does the noun belong to a niche or unpredictable declension?
Are you using the correct verb declension paradigm (Weak Verb Class I, Strong Verb, etc)?
Are you using the correct person/number, tense, and mood of the verb (with person/number agreeing with the verb's subject)?
Are you using the correct word order?
Some dictionaries such as Bosworth and Toller use old-fashioned Modern English, and this is easy to misunderstand. Do you have the correct Old English word for your intended meaning?
Are you using idioms?
Do your neologisms fit correct historical neologism formation patterns?