Ser vs estar

Both verbs translate as "to be", and English gives you no clue which one to pick. The starting rule works most of the time: ser for what something is - its nature, identity, origin; estar for the state it is in now - how it looks, feels or tastes at this moment, and where it is.

The interesting part is where both are possible, because then the verb changes the message:

So when a friend tries your feijoada and says Está ótima!, they are praising today's pot. A feijoada da tua avó é famosa describes its reputation.

Two memory aids

• DOCTOR for ser: Description, Occupation, Characteristic, Time, Origin, Relationship - Ela é minha irmã.
• PLACE for estar: Position, Location, Action in progress, Condition, Emotion.

Treat these as aids, not laws. Events take ser even though they are a "location": O show é no Maracanã.

Pairs that change meaning

• ser morto - to be killed (an event, a passive); estar morto - to be dead (the state).
• ser casado - to be married (your civil status); estar casado - to be married at present, a phrasing that hints the situation is new or might change.
• ser chato - to be boring or annoying as a person; estar chato - to be in an irritable mood.

Why English speakers get stuck

The habit is to pick one verb and use it for everything, and with adjectives it is usually ser: *Sou cansado, *Sou com fome. Ask each time: is this how the thing is, or how it is right now? And remember that a choice between é and está with the same adjective is often not an error at all - both are Portuguese, they just say different things.