An adjectival clause is a type of subordinate clause. It does the same work as an adjective in a sentence – it describes or provides more information about the noun in the main clause. An adjectival clause usually begins with the following words: that, when, where, who, whom, whose, which, why. Adjectival clauses often (but not always) appear in the middle of a sentence. Examples:

Christmas, which many people celebrate, can be (The main clause is “Christmas can be expensive”. The subordinate adjectival clause is “which many people celebrate”. The adjectival clause gives more information about Christmas.)

My cousins, who live in Johannesburg, are coming to visit us. (The main clause is “My cousins are coming to visit us”. The subordinate adjectival clause is “who live in Johannesburg”. The adjectival clause gives more information about “my cousins”. )

My grandfather remembers a time when people travelled in ox-wagons. (The main clause is “My grandfather remembers a time”. The subordinate adjectival clause is “when people travelled in ox-wagons. The adjectival clause gives more information about “a time”.)

The main reason why Puleng doesn’t like to share a room with her sister is her snoring. (The main clause is “The main reason is her snoring”. The subordinate adjectival clause is “why Puleng doesn’t like to share a room with her sister.” The adjectival clause gives more information about Puleng’s reason.)