Main clause
A main clause (or independent clause) contains a subject and a verb and expresses a complete thought.
Examples
Jenny was a teacher.
She lived in London.
These are both main clauses and also sentences.
Jenny was a teacher and she lived in London.
Two main clauses linked by the conjunction 'and'.
A main clause is not dependent on any other clause within a sentence.
When Jenny was a teacher...
This is no longer a complete thought as we do not know what happened when Jenny was a teacher. It is not an independent clause.
Every sentence must contain at least one main clause.
.
When Jenny was a teacher, she lived in London.
[time adverbial clause and main clause]
Jenny, who was a teacher, lived in London.
[main clause and embedded relative clause]
Jenny could be a head teacher if she lived in London.
[main clause and conditional clause]