English words in written mathematics

Charles Wells, The Handbook of Mathematical Discourse; it’s available as a PDF here. His site abstractmath.org may also be useful.

Again ('cos I've recommended it before) I can very warmly recommend getting your students to beg/borrow/buy and then read the excellent

Daniel J. Velleman, How to Prove it: A Structured Approach (CUP, 1994 and much reprinted, and now into a second edition).

From the blurb: "Many students have trouble the first time they take a mathematics course in which proofs play a significant role. This new edition of Velleman's successful text will prepare students to make the transition from solving problems to proving theorems by teaching them the techniques needed to read and write proofs." And in doing that, the most important thing, Velleman's book should get students to understand the lingo properly as they better understand what they are doing when they make a supposition here, draw an inference there, use a quantifier, etc.