Why is it wrong to say the word yahweh?

Answer 1

First of all, "Yahweh" is a non-Jewish pronunciation, foreign to Judaism, inaccurate, and invented because the tradition of the actual pronunciation of God's name (יהוה) has been lost.
Secondly, pronouncing it that way, while irritating to Jewish sensibilities, is not technically forbidden. And that's precisely because it's inaccurate. In Judaism there's a strict prohibition against correctly pronouncing God's unique name (Talmud, Sanhedrin 90a), except in certain rare circumstances (which don't exist today). During prayer and in blessings, we today use a lesser name of God, which is permitted to pronounce. And in ordinary conversation, we use a still-lesser word to refer to Him.