Is or are?

[Dilbertian]

When Mary or Jane is talking...

Or

When Mary or Jane are talking...

Which?

I think it depends on what the rest of it would say -
When Mary or Jane is talking, the rest of us should listen; When Mary or Jane are talking together, it is distracting for everyone else.

Are

imagine expanding the phrase

when Mary is talking or when Jane is talking, then ... ==> is
when Mary and Jane are talking together, then ... ==> are

Are

@EBearhug

I think it depends on what the rest of it would say - When Mary or Jane is talking, the rest of us should listen; When Mary or Jane are talking together, it is distracting for everyone else.

The second sentence doesn't make sense. It would read "When Mary and Jane are talking together"

Mary or Jane indicates only one of them is talking so it's "is". If both are talking, it would be "are" but that would require an "and" between the names..

Is

Is. Only one of them is taking so it’s singular.

is = singular
are = plural

So:
"When Mary OR Jane IS talking..."
(one of them is talking)

"When Mary AND Jane ARE talking..."
(both of them are talking)

Is

Either could be correct, depending on context.

If Mary and Jane were each a band, say, each with multiple band members, it could be correct to say ‘when Mary or Jane are speaking to the press I suggest they deal with the drugs allegations as follows . . ‘ but that’s the only type of context I can think of where ‘are’ would be correct.
Took some thinking too.

And I wouldn’t be putting that advice in writing

When Mary or Jane are talking is (imo) the correct option.

Is. Mary or Jane are singular. It would only be are if they were both talking.

Though I suppose technically it should be

"When either Mary or Jane are talking...."

Is.

@onelittlefrog

is = singular are = plural

So:
"When Mary OR Jane IS talking..."
(one of them is talking)

"When Mary AND Jane ARE talking..."
(both of them are talking)

That’s what I thought as well.

Yes agree. OR makes it singular so it's IS.

@BernadetteRostankowskiWolowitz

Though I suppose technically it should be

"When either Mary or Jane are talking...."

Won’t the word “either” change it to plural because you have a choice of two?

Definitely is, but if you said 'are' I probably wouldn't notice. Like the @parietal said the expansion test is helpful in situations like this- try writing the sentence out more fully. "When Mary is talking or Jane is talking..."

It should be is.

If you change the sentence to something else, it often makes it clearer eg 'when a fox or an owl is keeping you awake ...'

If you put are rather than is in that sentence, it implies they are doing it together, just as 'Mary or Jane are' does.

Won’t the word “either” change it to plural because you have a choice of two?

No, because only one of them is actually talking. "Or" means it is not both of them so a maximum of one person is talking. Therefore, it's "is" (singular) rather than "and" (plural).

Is

Oh yes, I misread itand got it wrong.

I'll go with my usual advice, if you can't work out how to write it that way, rewrite it entirely to avoid the issue.

It's 'are'.

If you were using the past tense you wouldn't say 'Mary and Jane was talking' so why would you say 'Mary and Jane is talking'