I understood the most of it, but not ..... word
( Either - every - each - neither )
Which word should I use and why ? If I used each is that mean I didn't understand all of it ?
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Sign up to join this communityEither means one of two, so, unless he only spoke two words, that is not right here.
Every, in this context, would mean that although you understood what he was going on about, there were some individual words that you did not understand. I think that is what you probably mean in this case. I should add that that does not imply that you have an inadequate grasp of English. Only a few minutes ago, I, a native BrE speaker with years of education, was reading an article in a non-specialist news magazine which used numerous words I had never seen before.
Each is definitely wrong in this context, but the reason is quite subtle. If you look at definitions of 'each' you will find something like "being one of two or more distinct individuals having a similar relation and often constituting an aggregate"(Merriam-Webster). Unless the person you heard was giving out a list of items that had "a similar relation", which the words in normal speech do not, each is not the right word here.
Neither is already negative. It is not correct in English to precede 'neither with 'not'. You will find an enormous literature on the subject of double-negatives in English. Sometimes they are OK. Sometimes in certain dialects but not others they are not OK. I do not believe that "Not neither ..." is ever right in English.