Invention No. 14 BWV 785 in B Flat Major

The last subject presentation starts with the bass.

The subject presented by the bass is imitated at the octave by the soprano. If the imitation starts before the subject ends, it is called a stretto.
In measure 17, Bach changes following harmonic reasons the soprano notes in red.
The imitation by the soprano ends in the third beat of measure 18.
The bass subject is modified starting in the second beat of measure 18. Probably to be able to finish the Invention in Bb major.