1. Устал- male УсталА-female, you need to choose «усталА» 2. Both not right in English. It means what kind of profession you have on work. Or what possession. Yes, this is instrumental case. For fun you can understand this many in way “you are instrument on your work, what kind of?” :)))) 3. Because you can ask “how” and you talk about HOW it was, and not talkin bout Maldives or darkness. And HOW is indefinite. “Это было прекрасно» -“ this was perfect”, THIS - what this? If you talk bout cake(торт) it will be «торт быЛ прекрасен», but You talk about something indefinite. If you talk right about darkness it will be «тьма былА в комнате» but it’s little bit Yoda style :)