When should I use SER or ESTAR?

SER and ESTAR both mean 'To be' in English, but choosing the right one can be tricky. The most useful way to remember the difference is to decide whether what you are describing has a permanent/inbuilt quality to it (use SER), or if it feels more temporary/momentary (then use ESTAR).
Two helpful acronyms can assist you in noticing the difference - DOCTOR and LoCo.To help you remember SER, think of the acronym DOCTOR: Descriptions, Occupations, Characteristics, Time, Origin and Relationships. These are lasting attributes which are unlikely to change. For ESTAR, think LoCo: Locations and Conditions. These attributes are likely to change, so we use the more temporary version of 'to be' in order to reflect that.