#russian beginner

Could somebody explain the rule with ‘быть + the instrumental case’? I am completely confused about when it is necessary. I know I wouldn’t use it in the present tense because ‘быть’ itself is not present, but what about other examples? Would I say ‘образование было моей мечтой’ or ‘образование было моя мечта’? Similarly, what happens with adjectives? E.g. how would I translate ‘he was sad’?

I have a question. for those Russian speakers among you! If I were to say ‘more useful skills’, would i be able to say ‘полезнее навыки’ or is that form reserved only for single things, and I’d be better to use ‘более полезные навыки’? :-)