I often hear these mistakes made by German speakers who are learning English.
They say: I already do this for several weeks.
The problem here is the tense. There are a couple of key words / phrases in the sentence which can help us to identify which tense we need. The key words / phrases are ‘already’ and ‘for several weeks’.
The sense here is of a repeated action which started in the past, is still being done now and will probably continue in the future.
Correction: I have already been doing this for several weeks.
Have fun learning English!
Louise