Improper Grammar
The following sentence has improper Grammar;

Tommy helped his uncle jack off a horse;

In the example above, the sentence is meant to convey that the the subject (Tommy) assisted his uncle (Jack) in dismounting from his horse; However, the sentence is confused with the lack of commas, which are meant to provide stops in the sentence, and therefore create clarity; Proper use of commas here would look as follows;

Tommy helped his uncle, jack, off a horse;

However, our sentence is not perfect yet, as the word "jack" is actually a proper noun, and All Proper Nouns Must Be Capitalized, Regardless Of Where They Lie In A Sentence; This is especially important here, as the word "jack" may be confused for it's slang meaning "to steal your shit" (ebonics); Hence;;;

Tommy helped his uncle, Jack, off a horse;

But grammar can be a tricky thing, and our sentence is still not complete; The closing of the sentence, "off his horse", also creates issues; This word choice may bring up images of cliché such as "Get off your high horse", and "Get off your high horse, motherfucker;" The simplest of word changes, "dismount", is also problematic, as the word "mount" often has sexual implications, which of course we want to avoid, especially since there is an animal in the sentence; Therefore, we must carefully select word choices which have no sexual implications;

Tommy helped his uncle, Jack, masturbate his horse until orgasm