I am a grammar nerd. I don't always get it right, but I aim to. I greatly dislike having bad grammar.
  1. "Your" welcome.
    People don't realize the difference between something that belongs to you and a contraction for you are.
  2. Than & then
    One compares. One shows a timeline.
  3. Not knowing the difference between common homonyms.
    Remember, homonyms are words that sound the same but have different meanings. Affect and effect. To, too, and two. Where and wear. There, their, and they're.
  4. When people answer this question incorrectly- "How are you feeling?"
    The answer is well. You do good and feel well
  5. Not using punctuation correctly... Or not using it all.
  6. Run-on sentences that are poorly constructed.