how the themes are presented differently in the Concerto Grosso and the Fugue.

In the fugue, the theme (subject) is presented as a whole, by the different voices
and parts. The four voices in the fugue take no precedence over each other, and take
turns to play the subject/countersubject. A fugue opens with one main theme, the subject,
which then sounds successively in each voice in imitation; when each voice has entered,
the exposition is complete; this is occasionally followed by a connecting passage, or
episode, developed from previously heard material; further "entries" of the subject then
are heard in related keys. Episodes (if applicable) and entries are usually alternated until
the "final entry" of the subject, by which point the music has returned to the opening key,
or tonic, which is often followed by closing material, the coda. In this sense, fugue is a
style of composition, rather than fixed structure. The main theme of a fugue is the subject.
The subject is then passed around to different instruments, mirrored, and often played in
conjunction with a counter-subject.
In a concerto grosso, in ritornello form, the theme (ritornello) is presented first as
a complete whole and then returns in fragments and variations after the soloist plays a
concertino. In the concerto grosso, only the concertino ever plays the solo and the roles
between the parts are fixed. the concerto grosso features a group of soloists rather than voices.. The rest of the orchestra (called "tutti" or ripieno") accompanies and plays in between phrases featuring the soloists.