"What is the difference between Ser & Estar" is one of the biggest questions that a Spanish teacher encounters. For those who have studied Spanish before, the difference lays in one being related to permanent situations and the other for temporary ones. That seems to be what most Spanish grammar books agree to. However that does not work!
I like to teach "Estar & Ser" rather than "Ser & Estar" because there are mainly and most imporantly 3 uses for Estar. If you can understand those 3 uses for Estar, then you can later discard every other use as belonging a the Ser verb. We will only refer to the three uses of Estar in this posting which are:
- Estar is used for the Progressive,
- Estar is used for Conditions, and
- Estar is used for the location of people and/or things (except those that happen).
That's all! Not clear enough? Let's try again:
- Estar is used of the Progressive. What is the progressive? This is when we say "one is doing something" or the verb to be plus the -ing ending form of the verb (i.e. I am speaking, you are eating, we are living). In Spanish we combine the use of estarplus the verb form that ends in -ando or -iendo (i.e. Estoy hablando, estás comiendo, estamos viviendo).
- Estar is used for Conditions. What do I mean? Well, the most common question in Spanish "¿Cómo estás?" really means "What is your condition?" Any time you or anybody wants to know "how is somebody" or "how is anything" what you really want to know is "what is the condition of someone or something." So if you say "How are your parents?" or "How is your class?" or "How is the weather today?" you will always be using Estar because you are inquiring about conditions: "¿Cómo están tus padres?", "¿Cómo están tus clases?" and "¿Cómo está el clima hoy?"
- Estar is used for the location of people and/or things. Except those that happen? Well let's get something clear. Do people happen? Does the table happen? How about your shirt? The answer to all three is NO WAY JOSE! Only events and activities happen or take place or occur. So we will use Estar for the location of anybody or anything except events or activities. So "Where is Maria?" or "Where is your house" or "We are in the United States" or "My friends are at the party" will all use Estarbecause they are not referring to events: "¿Dónde está María?" or "¿Dónde está tu casa?" or "Estamos en los Estados Unidos" or "Mis amigos están en la fiesta."
Finally, if you were to use the verb to be in any other context, it would be Ser and not Estar. This will be the topic of a future posting.
NOTE: For those who have studied Spanish before, notice how I never conditioned the use of estar to a permanent or temporary condition. It is not relevant or important if something is temporary or not.
Taken from Keeping Spanish Simple.