The one-word standby works as a noun, an adjective, or an adverb. It’s correct in the phrase on standby, meaning ready and waiting. By contention, the one-word form doesn’t work as a verb. When you need a verb meaning to stand beside or to be at the ready, use the two-word phrase stand by.
The plural of standby is standbys, not standbies. And because standby doesn’t work as a verb, standbied and standbying have no use. Stood by and standing by are the inflections of the phrasal verb stand by.
In April, he ditched it in favor of his old standby: the curveball. [Star-Ledger]
And will they go beyond the usual GOP standbys, like tort reform and selling health insurance across state lines? [Politico]
Residents in the Lefthand Canyon area were on standby to evacuate after automatic messages went to 340 phone lines warning them of the fire Sunday night. [Denver Post]
That confidence was hard-won and no inflation-targeting central bank can stand by and see it frittered away. [New Zealand Herald]
Mohamed Sabual used to stand by silently as he heard classmates and other youths mocking Muslims. [Calgary Herald]
Stand by for a glimpse of one of the brightest stars of the week. [Telegraph]