This is the 30th most frequent German word.
sei
The German word "sei" is the subjunctive form (Konjunktiv I) of "sein" (to be).
Here, 'sei' is the imperative form of the verb 'sein' (to be), giving a command or request in the second person singular.
Sei höflich, wenn du mit Kunden sprichst.
Be polite when talking to customers.
'Sei' in this context represents the subjunctive mood, used to express a statement that is reported or uncertain.
Man sagt, sie sei sehr begabt.
It is said that she is very talented.
In this sentence, 'sei' is used as a subjunctive form in indirect speech to convey reported information.