If you say I have EATEN A WHALE You are simply stating the fact and you are stating it in a PRESENT PERFECT TENSE. It does not matter WHEN or WHERE.... you ATE/EATEN that WHALE ONLY that it was done. If you say I have never eaten A whale, again you are only stating the fact, that you have not done it but because you are ADDING NEVER you are also telling us that this was... well 0 TIMES in your life :P

If this is all one sentence yes than it is weird hehe I am working here for 2 monthS NOW. The now part is kind of important because English has a thing called TENSES and when you say that you have been doing an activity like working, you can use PRESENT CONTINUOUS tense to explain that. But you can also use a different TENSE like I have been working here for a month. Its a PRESENT PERFECT CONTINOUS Perfect is used when just giving a piece of information or rather when we are interested in the end result more opposed to WHEN it has happened. In your case you are telling someone you are WORKING HERE for 1 month. :D WORKING HERE 1 month 3 pieces of information and WHEN is not important. Sort of :D

Don't need to FORGET and MUST not FORGET are 2 totally different meanings. DON'T NEED TO FORGET 1) Is almost never used, but still sometimes it is: Example: You saw some BAD thing happen and your friend says to you: "You DON'T NEED TO FORGET - remember it and learn from it....for instance... 2) MUST NOT FORGET is the opposite: You saw something bad happen, but YOU MUST NOT FORGET IT, because it is really important to you for instance. So must is usually also used with an exclamation! I could get into it more grammatically with MODAL verbs and so...but I think there is no need for that. And as for NOR... nor is used instead of for instance you DON'T NEED to do THIS + YOU DON'T NEED to do THAT, so basically NOR replaces the +/and part of it, when you want to mention two same things that you DO NOT NEED to do (and remember it is only used in negative sentences not in assertive or affirmative sentences)

2 things 1. HANG ON TIGHT and HOLD ON TIGHT are both IDIOMS But they have a different although similar meaning. a) HANG ON TIGHT means (hold with your hands for dear life :P...) b) HOLD ON TIGHT (means wait here! while I get back usually - so to wait in one place) 2) And as for TIGHT or TIGHTLY actually both are technically correct, and tightly is more grammatically correct because it is an adverb. However, because this is an IDIOM (means it's just how the sentence is used) the correct answer is: a) hung on tight

In general, they should of course. However, because of the way how they are basically not being thought their language, but rather just speak it and how most don't pay attention to the grammar and the rules in general, I would say the following. It is not about the native speaker, but rather about how well can someone teach. If you know the language well, then someone who can teach well is better, than someone who doesn't know how to.