Albert Barnes on Mark 1:21

And they went into Capernaum. For the situation of Capernaum, see Mt 4:13.

Straightway. Immediately. On the following Sabbath.

The synagogue. Mt 4:23.

And taught. In the synagogue, the presiding elder, after reading the Scriptures, invited any who chose to address the people, Acts 13:15. Though our Saviour was not a priest of the Levitical order, or an officer of the synagogue, yet we find him often availing himself of this privilege, and delivering his doctrines to the Jews.