Yes, this is she. Who's calling? [duplicate]

Solution 1:

Your instinct is a good one. You may hear "Yes, this is she," if you call a professional office, but most American speakers would say "This is Mary" or, "Speaking" to avoid sounding stuffy.

"Hey, it's me," is a given, due to the informality of hey, probably.

"They believed that the thief was I," would probably be avoided by saying, "They believed that I was the thief."

Solution 2:

It seems that you have a very good ear for colloquial English. I wish my ear for the languages I've studied was as good.

The grammatical construction you're asking about is the "predicate nominative" -- and as you already noted, it is very common to use the objective case even when the subjective is called for.

To look at your specific examples:

• "They believed that the thief was I." (This sounds stilted and odd -- more so since it is so easy to say "They believed that I was the thief.")
• "Hey, it's I." (I guess this is technically correct. Or, at least I can't identify a reason that it is wrong. But it sure sounds wrong -- perhaps because of the informality of the word "hey" and the contraction "it's". To my ear, a more formal rendering of "Hello, it is I" doesn't sound nearly as disturbing as "Hey, it's I".)
• "Yes, this is she. Who's calling?" (This is actually pretty common. It sounds a bit formal -- but not weird.)

To answer your question: Yes, it will sound unnatural if you use the subjective case in casual conversation (as it would if I used it).