I was wondering how a passive sentence like:
トムさんに話しかけられた。
Tom started talking (to me).
could be changed to express that it was the first time that it happened. My attempt was:
トムさんに話しかけられたのは、初めてだった。
Is it gramaically correct? ( Should 話しかけられる be in the plain form or in the informal past?)
Does it sound natural, or is there a better way to say it?
Your attempt, トムさんに話しかけられたのは初めてだった is a perfectly correct Japanese sentence. You can also say トムさんに話しかけられるのは初めてだった, which is almost the same in this case. But note that ～るのは初めてだった and ～たのは初めてだった are not always interchangeable. For example, you have to say トムさんに会うのは初めてだった if "meeting Tom" happens relatively in the future of the time this statement is about.
(If you just want to use 初めて with the original sentence in the passive voice, the simplest way to do so is トムさんに初めて話しかけられた, of course.)
For reference, here are the hit results from BCCWJ (online corpus):
I did the following two Google searches:
Looks like it's more favored to say:
トムさんに話しかけられたのは、初めてだった。
than to say:
トムさんに話しかけられるのは、初めてだった。
Another way to express the same meaning would be:
トムさんに初めて話しかけられた。
