Psalm.
A numbered musical piece might be called a concerto, an opus, or a composition. It could also be called a symphony.
The term for a second melody that accompanies the main melody in a musical composition is called a "counterpoint."
Study
instrumental
Usually, they are called a musician.
The musical composition that usually precedes a fugue is called an exposition. In this section, the main theme or subject is introduced, typically in one voice, and is subsequently imitated by other voices entering one after another. This establishes the thematic material that the fugue will develop and explore through various contrapuntal techniques. The exposition sets the stage for the intricate interplay characteristic of a fugue.
A group of Musical Instruments is called a consort.
A written composition is usually referred to as an essay, article, or piece of writing. It can also be called a composition or text, depending on the context and format.
Basso continuo
I think it is a motive.
Two singers are a duo.They might sing a duet, a musical composition for two performers.
It's called an etude - i.e. A musical composition written solely to improve technique.