[Nagoyankee]

1. There is usually only one way of writing an English-stemed word in katakana.

warning = ウォーニング　Not that this word is often used in Japanese, though. Let's pretend your family name is Warning. It will be written none other than ウォーニング.

2. da and ga are NEVER interchangeable. Da is used in casual speech to mean "desu" = to be. Ga is a subject marker in a sentence.

Shinobi da. = If you mean 'ninja' by shinobi, you will in dramas hear "Shinobi da." to mean either "I'm a ninja" or "That's a ninja". You can never say "Shinobi ga." Something has to follow the ga, as in "Shinobi ga modotte kimasen" = The shinobi hasn't returned.

3. The only times you use the ー mark is when you elongate the syllable right in front of the ー. It's not you have the choice of using it and not using it.

In the case of モーニング, it's always モーニング. You will sound more foreign than you might think if you say モニング.