#english_language 

Essential clauses are clauses that are essential to the meaning of a sentence.

Clauses beginning with *that* are always essential to the meaning of a sentence and should **not be set off by commas or anything else**, unlike [[Non-essential clauses|non-essential clauses]].
Clauses beginning with *who* can be either non-essential or essential, with slight differences in meaning.

**THESE CLAUSES ARE SET OFF WITH "THAT"**:
Farm animals *that* were introduced to ....