Support and Sustain are synonyms, so the difference is slight. If you are speaking about a person you might say something like; "I support his new career choice." but you would never say; "I sustain his new career choice" However if you are talking about a building for example you might say; "The walls support the heavy ceiling." or "The walls sustain the weight of the ceiling" In this case the words can be exchanged without much difference in meaning. Sometimes you might say something to confirm or agree with another person; "I support his statement about the car accident." again you would never say; "I sustain his statement about the car accident" Hope this helps some, in most cases you can use the word support and the meaning will be almost the same.