[derleth]

Grammatical deficiencies are symptoms of conceptual deficiencies.

Assuming you actually know what grammar is as distinct from usage, and don't try to claim someone is less intelligent just because they have a different style of usage from you.

For example, using a split infinitive is not indicative of anything other than the style that person prefers. Same with the passive voice more often than not (assuming the person doing the marking actually knows what the passive voice is, as most of the people who get aggrieved over it do not).

And, of course, if you think someone who speaks a dialect distinct from the prestige dialect must be less intelligent, that says terrible things about your own intellect.