1. Honestly, I would use both Teachers?
2. Your goal is to be clear. "Plainly" means "without frills", so to speak: since we don't expect any "extravagance" in a definition, "clearly" is - clearly - the better choice.
3. Think of "alien" as "from another world, that you cannot understand". If something is alien to you (like, jealousy), you simply don't understand how one can be jealous. As for "foreign", it just means that it is not familiar to you eg. foreign accent, food etc.
4. You can exceed a limit or an objective, but not someone else's results.