Pages autocorrects with wrong spelling
I've been noticing lately that when I write in Pages (version 5.5), oftentimes I will spell a word correctly, but after I hit the spacebar and move on to the next word, Pages will change the spelling of my correctly-spelled word to something totally incorrect that I didn't type. Sometimes it will repeat the last letter of the word, other times it just adds random letters to the middle of the word. Is there a way to stop this?
Also, when I want to use a word that isn't in the Apple dictionary, Pages will suggest a correction, which I decline, which Pages then changes my word to. Every time I go back and spell the word the way I want to, it keeps changing the word back to the one it suggested as soon as I hit the space bar. Is there an easier way to get it to accept my declining its suggestion than adding every word to the dictionary? (I'm a nursing student and there is a lot of medical terminology Pages doesn't recognize)
MacBook Pro, OS X Yosemite (10.10)