Conversation so far:
How did he start his teachings?
Namdev's songs were composed to be melodious and carry a spiritual message.
They built on one among the many ancient Indian traditions for making music and singing.
What were the Bhajans?
Namdev composed bhajans, that is songs meant to be sung to music.
A Bhajan literally means "a thing enjoyed or shared.
Who did he teach the Bhajans to?
The songs and music that went with Namdev's works, were usually transmitted verbally across generations, in a guru-sisya-parampara.
Did he have help composing the Bhajans?
CANNOTANSWER.
Are there any other interesting aspects about this article?
In Namdev's songs, the dominant pattern is Caturasra, or an avarta with the 4x4 square pattern of musical matras (beat).
where there any people who opposed Nandev and his teachings?
Callewaert and Lath state that, "each single song of Namdev is a musical and textual unit and this unit is the basis for textual considerations".
when was the Bhajans created?
CANNOTANSWER.

Question: were the Bhajans useful?
Self-contained rewrite: Were the Bhajans useful literary works?