maintain
通过持续努力防止水平或状态下降
sustain
提供支撑使之持续存在,强调「供养」
The company has managed to maintain its market share despite intense competition.
尽管竞争激烈,该公司仍设法保持了其市场份额。
The emergency food supply was just enough to sustain the survivors for three days.
紧急粮食供应刚好足以维持幸存者三天的生存。