Non-defining relative clause (NDR)

NDR are not essential for understanding the sentence, they just give some extra information. They are a useful way to combine two short sentences into one long sentence.

(e.g.) This model, which reguires five parameters, was adapted from the one published by Mitchell et al..

We use commas with NDR. We use pronouns whether they are subject or object. We can still omit the pronoun and ‘be’ if they come together (, which was used …). We always use ‘which’ for things and ‘who’ for people (occasinonally ‘whom’).

In DR, the pronoun always refers to the noun which comes immediately before.

BUT 

In a NDR, the pronoun can refer to the noun immediately before or the whole of the preceding sentence. Be careful!

(e.g.) Mitchell et al. have produced a model, which contains five parameters, that can …

(e.g.) Designing these models, which can take several months, is not an easy task.

(e.g.) Designing a model to accurately simulate network traffic, which can take severak months, is no wasy task. → What taskes severakl network? Designing the model or simulating? Network traffic?

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