“because.” See M. Fox, The JPS Bible Commentary: Ecclesiastes.
“also taught.” This is likely in contrast with just writing. While “also taught” seems somewhat awkward, we don’t really know how much ancient Sages were expected to verbally teach (cp. Tremper Longman; The New International Commentary on the Old Testament: Ecclesiastes).
“proverbs.” The Book of Proverbs notes that Solomon spoke and wrote many proverbs, and 1 Kings 4:32 says Solomon spoke 3,000 proverbs, but there are only some 900 proverbs in Proverbs, and some of them are not from Solomon.