ジョンがドアにペンキを塗っていた。

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

door (Western-style)

at (place, time); in; on; during; to (direction, state); toward; into; for (purpose); because of (reason); for; with; by; from; as (i.e. in the role of); per; in; for; a (e.g. "once a month"); and; in addition to; if; although

paint

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to paint; to plaster; to lacquer; to varnish; to spread; to smear