1. Principle and Principal - An easy one to distinguish if you think about it, which seems like the catchphrase for most of the mistakes out there. To clarify:
- Principle - An accepted or professed rule of action or conduct. For example, "One of my principles is not to kill; you are making it hard to adhere to with your bad grammar."
- Principal - As an adjective it is the first or highest in rank, importance, value, etc.; chief or foremost. For example, "My principal principle is not to lie".
2. Effect and Affect - Two very different words that are also easily misused. To clarify:
- Effect - It is something that is produced by an agency or cause, a result, a consequence. For example, "This blog post has had a deep effect on me."
- Affect - This means to act on; produce an effect or change in. For example, "I have been affected by a very wise blogger".
That said, I have probably made mistakes in this post (Murphy loves me). Feel free to correct me and post your own grammar nazi advice in the comments.
All the definitions are taken from dictionary.com. Just sayin'.