I often hear these mistakes made by German speakers who are learning English.
They say: I have a new boss. She is in the department since the beginning of September.
Since is the keyword in this sentence and tells us which tense we need: Present perfect tense and not present tense (as in the example above.)
Correction: I have a new boss. She has been in the department since the beginning of September.
As an alternative we can remove the word ‘since’ and use the past simple tense.
Alternatively: I have a new boss. She started in the department at the beginning of September.
Have fun learning English!
Louise