Questions & Answers about Hoy estoy cansado, pero igual estudio español.
Spanish uses estar for temporary states or conditions and ser for more permanent characteristics.
- Estoy cansado = I am tired right now / today (temporary condition).
- Soy cansado would sound like “I am a tiring person / I tend to get tired easily,” a more permanent trait, and is rarely what you want.
Since being tired here is clearly a temporary, current state (“today I’m tired”), estar is the correct verb.