应当【yīngdāng】

应当 states a duty and goes before the verb. Laws and regulations use it for "shall", and formal writing uses it for what someone ought to do. In speech the same idea is 应该.

Legal text often shortens it to 应, especially in a run of clauses.

应当 or 应该

应该 does two jobs: duty, and a confident guess about what is probably true. 应当 does only the first. For a guess it sounds wrong.

• × 他八点出发的，现在应当到了。 → 他八点出发的，现在应该到了。

The negative

不应当 says something ought not to happen and is used in argument and comment. A legal prohibition uses 不得 instead.