If all swans are white, and this bird is black, then this bird must not be a swan.
However, if this bird is a swan, and this bird is black, then not all swans are white.
How can I hear you if you mumble?
If you buy this computer today, we will give you a memory upgrade to 16 GB for free.
If you need to drive to Springfield today you'd better go the long way, there's construction on the Expressway.
"Miss Pipperidge, if you'll be mine, I'll be yours for the rest of my life."--From a show tune
Colloquial and British: "He’s been in the army four years, he wants a good time, and if you don’t give it him, there’s others will, I said."--T. S. Eliot, "The Waste Land"
"If wishes were horses, then beggars would ride."--Saying
"If the interruption to the flow of the sentence is but slight, the writer may safely omit the commas."--Strunk & White, "The Elements of Style."
"If I have seen further, it is by standing on the shoulders of giants."--Sir Isaac Newton