His main focal points in research and teaching are African linguistics, sociolinguistics, grammaticalization theory and language contact and discourse grammar.
The grammaticalisation theory, which deals with the changes in grammar, and to which he contributed 7 books and numerous articles, is his main focal point.
He carried out 25 field research trips to Ghana, Togo, Kenya, Tanzania, Uganda and Namibia as well as lecturing trips to Australia, Austria, Brazil, Estonia, Ethiopia, Finland, France, Italy, Japan, Kenya, Mexico, Namibia, the Netherlands, Poland, Spain, South Africa, South Korea, Sweden, Tanzania, the United Kingdom and the United States.
From 1997 to 2000 he was president of the Committee for World Congresses of African Linguistics and in 1999 he became a full member of the North Rhine-Westphalia Academy of Sciences.
In 2008, the Ministry of Education, Science, and Technology, South Korea, appointed him a Distinguished World-Class Scholar, and in 2009 he received the Lifetime Achievement Award of the Evolutionary Linguistics Association, Brussels.