Portuguese/Contents/Ser and estar
One of the simplest constructions in any language is the ability to express a relation of equality between nouns or nouns and adjectives. This construction is often called copula, the Latin word for 'link', 'bond'. For example, consider the following statements:
- John is tall.
- Mary is a dentist.
- Robert is happy.
These sentences link together the nouns (in these cases, 'John,' 'Mary' and 'Robert') with an adjective or noun (respectively, 'tall,' 'dentist' and 'happy'). In English, this is achieved by using the helper verb 'to be'.
In Portuguese, as in Spanish, we have actually two verbs which serve that role. Let's examine them, each in turn:
The first verb is ser, which comes from the old Latin verb esse, which meant 'to be'. It's conjugated (in the Present Indicative tense) as follows:
eu sou tu és ele/ela/você é nós somos vos* sois eles/elas/vocês são
The second verb is estar, which comes from the old Latin verb stare, which meant 'to stand'. It's conjugated (again, in the Present Indicative tense) as follows:
eu estou tu estás ele/ela/você está nós estamos vós* estais eles/elas/vocês estão
*Not commonly used
Using ser and estar- Usando Ser e Estar 
Ok, now that we know the verbs, when do we know how to use them? What is the difference between ser and estar? The answer is actually quite simple, but it requires attention, especially for speakers of Spanish: the instances in which we use ser and which we use estar in Portuguese and Spanish are almost, but not always, the same.
Remember when I said that the verb ser comes from esse and the verb estar comes from stare? Well, there are two words in English which also come from these two Latin verbs. From esse we have English essence, and from stare we have English status. So we use ser when we want to describe the essence of something, or how a thing is always; and we use estar when we want to describe something's status, or how a thing is at the moment.
Let's see some examples, then, to fixate:
- O sol é amarelo.
- The sun is yellow.
- Meu nome é Jim.
- My name is Jim.
- Nós somos Brasileiros.
- We are Brazilian.
In each of these cases, the speaker attributes to the subjects of the sentence characteristics that are intrinsic to them. People don't change names that often, or nationality. It is part of their identity. Also, things are generally only one color, with a few notable exceptions like chameleons or traffic lights. All these cases use the verb ser.
- A cerveja está gelada.
- The beer is cold.
- Eu estou doente.
- I am sick.
- O livro está sobre a mesa.
- The book is on the table.
In these cases, the phrases describe temporary situations: beer can get warm, sick people get well, and books can be picked up and carried around. Therefore, these are cases where we use estar.
Notice that there are some cases where one could use ser or estar equally well; but in these cases, as expected, the meaning changes when we use one or the other case. Watch:
- A moça é bonita.
- A moça está bonita.
Both sentences above translate to 'the young woman is pretty'; however, in the first case, we mean that the young woman is naturally pretty, like Anna Kournikova. In the second case, we mean that she is prettier than usual -- typically because she put on some nice clothes, got a haircut, or otherwise accessorized herself. She may or may not be pretty normally, but we want to call attention to the fact that she is prettier due to some temporary condition.
- Tony está doente.
- Tony é doente.
Again, both cases translate to 'Tony is sick'. However, the first one explicitly says that Tony isn't usually sick, but is right now. The second one means that Tony is sick now, was in the past and will be in the future -- generally, this has connotations that Tony is mentally sick.
The location of things 
When we want to describe the location of things, naturally we use the copula. In this case, we use ser when something generally can't move around, and estar otherwise. Spanish speakers should be aware that this is different from Spanish, where estar is used everywhere. Some examples:
- O carro está na garagem.
- The car is in the garage.
- Os bilhetes estão sobre a cama.
- The tickets are on the bed.
- Eu estou na estação.
- I am in the station.
- O hotel é na praia.
- The hotel is on the beach.
- A casa é mais adiante.
- The house is further ahead.
- O castelo é em Portugal.
- The castle is in Portugal.
In the first three examples, we have items which are expected to be able to be in a wide variety of places over time, so their location is not intrinsic to them: estar is used. In the last three examples, we have subjects which are generally immobile, and therefore have an intrinsic location; ser is the correct option then.