Unknown and known form are text tools. They have the same meaning so far that they refer to the same thing. But by using unknown form you indicate new information, while known form indicates that the information is something obvious for the reader/listener - either from the text or from the context.

In this phrase: Jeg tog bussen ind til kontoret. (I took the buss to the office), the use of known form of bussen tells the reader that the buss is the one connecting home and office: the relevant buss. The term "known" is slightly misleading: The reader might not know the buss, but at least he should be able to figure it out.

In this phrase: På vej til kontoret så jeg en bus fuld af politifolk i kampuniform. (On my way to the office I saw a buss full of policemen in batledress) the use of unknown form tells the reader that the buss is something that breaks the pattern - a point of interest.

Choose unknown form (article: en/et/nogle) or known form (ending -en/-et/-ne) of the words in the drop down list.