Assessing Formality via the Root of the Word

The pattern works pretty well for the often mentioned animal/food pairs. informal animal:sheep

The list of pairs or triples is endless. Just a handful:

• land/country/nation
• friendly/amiable
• drink/beverage
• wound/injury
• room/chambre
• woods/forest
• child/infant/juvenile

This is good rule of thumb, but isn't absolutely perfect. There's hips (OE) and haunches (hips/legs, OFr) and ham (leg (of ham) OFr), where hips is the most formal of all of them. Ward (OE)/guard (OF), corner/angle, neck/collar, harvest/autumn all seem of the same formality

Another difference, in addition to formality, is the Anglo-Saxon is usually more concrete, the Romance more abstract.

The pattern also works well for legal or medical terms: theft/larceny, hand:manual, knee:genuflect, head:capital (and then Greek neologisms are created for even more synonyms).

But these technical medical terms are more likely to be technical neologisms well after 1066, by scientifically minded individuals rather than social language sharing via an invasion.