staid, stayed

Staid or staid  ( rhymes with stayed, sometimes pronounced more like "sted" ) describes a solid, steady, or reliable character--
He has always been too staid to do anything so capricious.
He thought she was staid and boring; she thought he was flighty and irresponsible.

Stayed or stayed ( rhymes with staid ) is the past tense ( yesterday, or some time ago ) of the verb ( action word )  "stay", meaning to remain or continue--
He stayed at work until after 7 o'clock.
We stayed dry under the umbrella.

Now that you know that, you can say--
"He stayed with the same firm for many years, which proves him to be staid and dependable."

Note--not to be confused with "stead" (pronounced sted ), meaning the place taken by a substitute--
He sent an employee in his stead, authorized to close the deal.
"Stead" is also an old-fashioned word for repute, surviving in the phrase "stand in good stead"--
If you take this course, you will stand in good stead with a prospective employer.
"Instead" ( one word ) means as an alternative or substitute--
Let's have pizza instead of burgers.