1 & 2: Yes, you are quite right about the distinction. 3: The word you are thinking of is 'perpetual' (no letter 'c' in it). But no, you wouldn't use 'eternal' to describe noise, and I wouldn't say that perpetual and eternal are interchangeable in most cases. 'Constant' would be the best word to use here.

You are completely correct. "Eternal" is a bit religious or poetic in use. There is an "eternal flame" at the tomb of John F. Kennedy.

Thank you, Richard! It is very helpful! One thing I am still confused about: I didn't include sth.--I left sth. out versus I have left sth. out. The grammar tells me that using the present perfect tense means the past action has an impact on the present. Is it why I need to use "have left out" here? Sometimes, it is hard for me to draw the line. Thank you!

Good question Kailin. I just want to offer a couple of suggestions regarding your sentence ``Please tell me whether I understand them right or if I leave out anything or how you will use them.` After the verb ``understand`` you can ask the question ``how``, so that means that the word describing how must be an adverb. ``right`` is an adjective and there is no adverb ``rightly`` so it would be better to use the adverb ``correctly``. If you did not include everything, then you ``have left something out``. ``will use them is correct, however, it would be better to say ``how you would use them``, because ``would`` is used to express something that is in the future but may or may not be ``used``. Please tell me whether I understand them CORRECTLY or if I HAVE LEFT ANYTHING OUT or how you WOULD use them.