Sentence:  Much of the information about a group can be gleaned from a study of its <WSD>subgroups</WSD>. For these reasons it is important to study the <WSD>subgroup</WSD> structure of the almost simple groups, and in particular their maximal <WSD>subgroups</WSD>. 
Meaning of the word marked with <WSD> tags:  A subset "H" of a group "G" that is itself a group and has the same binary operation as "G".