I was under the impression that we can't use a comma before 'if' for the following sentences:

- Please let me know if you need any further information.

- We will get wet if it rains.

When using Cambridge Advanced Learner's Dictionary though, the following sentences were used when looking up 'would' and 'could'.

- I wouldn't worry about it, if I were you.

- We could go for a drink after work tomorrow, if you like.

Why is a comma used before 'if' in the would/could sentences, but not in the first two sentences?

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Comments  (Page 2) 

Rule/example xD Very nice.