therefore and therefor

This Denglish mistake is the most problematic and persistent mistake I have had to deal with during my time correcting German-English texts. "Therefore/therefore" is very frequently used incorrectly by German speakers, and it seems to be difficult to remember the correct words to use.

There is only one correct way to use "therefore".

Therefore:

"Therefore" is the best friend of "because" and should be used when the two clauses of a "because" sentence are flipped around.

Example: 

He was unable to play football because he was injured.

He was injured, therefore he was unable to play football.

Rule: If you can't flip it to make a "because" sentence, don't use "therefore".

Therefor:

This word is not used in modern English. Modern spell checkers will underline this as a mistake. Don't use it. 

Origin of the confusion:

For example, you want to say:  

Ziel des Projektes war, herauszufinden ob Coca Cola und Pepsi in Blindversuchen von einander unterschieden werden können. Dafür wurden 200 Versuchspersonen unbeschriftete Becher mit Coca Cola oder Pepsi gegeben.

The correct English translation is:

The aim of this project was to find out if Coca Cola and Pepsi could be correctly identified in blind trials. To do this, 200 test subjects were given unlabelled cups containing either Coca Cola or Pepsi.

Other correct options: 

...identified in blind trials. To this aim, 200 test subjects...

...identified in blind trials. To investigate this, 200 test subjects...

...identified in blind trials. This was done by giving 200 test subjects...

...identified in blind trials. In this study, 200 test subjects...

...and many more. 

Or nothing:

...identified in blind trials. Two hundred test subjects were...

This abstract is a good example of the correct use of "therefore" and "to do this". http://www.nature.com/nature/journal/v267/n5608/abs/267281a0.html

Still not convinced?
Many people simply don't believe me. Look up "to do this" (with quotation marks " ") on Google Scholar and note how often it is used in this context by American/Canadian/British/Irish/Australian authors. You will not find a paper by a native English speaker that uses "therefore/therefor" to mean "dafür", only papers by other non-native speakers. 

But google told me...
If you google "therefor" and "dafür" you will get some google hits. In Shakespeare's works you can find "therefor" used for this purpose, but it is not used like this in modern English. Unless you are writing in Shakespearean English, do not use it.

Incorrect:
"thus" is also wrong and very old fashioned.