TITLE: Mathematical writing: why should we not use the phrase "we have that"?
QUESTION [34 upvotes]: In Knuth's Mathematical Writing, he writes on page 2, at number 8:

Don't omit "that" when it helps the reader to parse the sentence.

Bad: Assume $A$ is a group.
Good: Assume that $A$ is a group.

The words “assume” and “suppose” should usually be followed by “that” unless another “that” appears nearby. But never say "We have that $x=y$," say "We have $x=y$."

The last bit seems to contradict the previous sentences; is it just an exception? What makes "we have that" so bad?

REPLY [6 votes]: "We have $A$ equals $B$" is wrong. (The sentence has two competing verbs: "have" and "equals".)
"We have the equality of $A$ and $B$" is fine.
The word "that" is needed in the first sentence to convert "$A$ equals $B$" into a noun clause, while its intrusion into the second sentence would be quite wrong. Thus "we have that $A=B$" is right or wrong depending on how you read "$A=B$". It seems that Knuth adopts a prescriptive position over English but a liberal one over mathematics, since he surely interprets "$A=B$" as "$A$ equals $B$" in other contexts.