В каких случаях нужно употреблять инструментальный падеж когда...

...употребляете подлежащее и прилагательные? Есть ли простое правило? Например, если бы я сказал, "Я буду юристом". В большинстве случаев, я правильно угадываю но не знаю общее правило.

"Я не могу потому что ты слишком подозрительный"

Does it have anything to do with changing states? Like if the condition (the adjective) is not a permanent state of being, then you would use the instrumental? Or does it have to do with present and future tenses specifically?

In present (with the verb to be) only nominative is used. In the Past and Future instr. is usually used, nom. is rarer. The inf. and imper. requires instr. only.

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Does it have anything to do with changing states? Like if the condition (the adjective) is not a permanent state of being, then you would use the instrumental?

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No, it doesn't.