What I found till now is:
One approximation to get the root word:
- Remove -lein, -chen, -erl, -ette from the end of the word.
- Replace ä with a, ü with u, ö with o.
If the word is a "zusammengesetztes Hauptwort" only replace the letters of the "Hauptwort" at the end.
- Add -e, -er, -en or nothing to the end of the word.
This does not cover all possibilities.
Examples:
Kärtchen to Karte
1.) Kärt (After removing the -chen)
2.) Kart (After replacing ä with a)
3.) Karte (After adding -e)
Häuschen to Haus
1.) Häus (After removing -chen)
2.) Haus (After replacing ä with a)
Kräutergärtchen to Kräutergarten
1.) Kräutergärt (After removing -chen)
2.) Kräutergart (After replacing ä of the last Hauptwort with a)
3.) Kräutergarten (After adding -en)
Märchen to Mär:
1.) Mär (After removing the -chen)
Stiefelette to Stiefel
1.) Stiefel (After removing the -ette)
Weckerl to Wecken
1.) Weck (After removing the -erl)
2.) Wecken (After adding -en)
Eichhörnchen to Eichhorn
1.) Eichhörn (After removing the -chen)
2.) Eichhorn (After replacing the ö with o)
An example where this approximation is wrong:
Mädchen to Magd:
1.) Mäd (After removing the -chen)
2.) Mad (After replacing the ä with a)
3.) You cant get to Magd by adding something to the end.