Final answer:
To form a relative clause from the two sentences, we combine them into 'The pen, which is expensive, doesn't write well.' by using 'which' to relate the expense to the pen and provide additional information.
Step-by-step explanation:
To transform the sentences "The pen doesn't write well. It is expensive." into a sentence with a relative clause, we begin by identifying the subject that both sentences have in common, which is the pen. By doing so, we can combine the two sentences into one by using a relative clause to provide additional information about the pen without repeating the subject. Here’s how you can combine them into one sentence with a relative clause:
The pen, which is expensive, doesn't write well.
In this sentence, 'which is expensive' is the relative clause providing more details about the pen. We use 'which' because it refers to a thing and adds non-defining information to the sentence. Remember, relative clauses can add both essential details (defining relative clauses) and non-essential details (non-defining relative clauses) about the subjects to which they refer.