Interesting one! The meaning and weight of the words "pity" and regret" will all depend on the sentence and overall feel of the writing. Both are strong emotions...pity can be used with cynicism but it can also simply be pity. For example, "When I saw him leaving the hospital, his hand clutching to the frame, I regret everything that had happened; But most of all I felt this unyielding sense of self pity. I hate myself for being so selfish". or "When I saw him leaving the hospital, his hand clutching to the frame, I pitied him so much; But most of all I felt this unyielding sense of regret. I hate myself for being so selfish". I may have just complicated matters :-) But hopefully you can understand?

It does sound wrong to the native ear, and it makes no sense. You can pity a person, or an animal, but not a situation. I'm not quite sure what you are trying to say here. Do you mean 'regret' in the sense of wishing that you hadn't done something or wishing that something had never happened? This has nothing to do with the verb 'to pity'. The nearest you can get is an expression such as 'It's a pity that you did that' or 'It's a pity that this happened'. Is that what you want to say? I also wouldn't agree that 'regret' is more emotionally powerful than 'pity'. The verb 'to pity' is usually quite personal and heartfelt, whereas regret can be quite detached and formulaic. 'Regret' is used the whole time, in writing and in speech, by business people and politicians and it means nothing at all - it's just a formula.

I agree with Su.Ki. Dmitriy, if you wish to use the noun "pity" with another noun, you can try, "His predicament fills me with pity", or "My present hardship fills me with self-pity", but these may not be what you want to say.

Yes, it is possible (albeit (хотя и) rare) to pity something instead of someone. If you google, you can find examples of such use. Here are a few I have found: "Pity the land that has no heroes. Unhappy the land that needs heroes". // "Do we pity the environment? Fear of environmental destruction causes us to pity and protect nature rather than oppose it." // "it's a tractor made by 19th-century Viennese manufacturer Hofherr-Schrantz. Pity the earth under its wheels." // "I pity the forest that produced all that wood."