¿Cómo dices esto en Inglés (US)? 71. those are nice chairs. I’ve seen some good movies recently. why in the first one should’t use “some”?And why second one we have to use “some”?

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You can use OR not use 'some' in BOTH of them.

It can mean either "more than one", or, it can make the rest of the phrase more emphatic.

In the first one - 'Those are some nice chairs!' - since you said "those" I probably already know that there are more than one, so, it sounds emphatic, as if you are saying "Those are really nice chairs!" or "very nice chairs". just "Those are nice chairs" sounds perfectly fine also.

You can use it with singular things too: "That is some girlfriend you have!", it means either I really like her or I really don't like her. From context you would know which. "Donald Trump is some President!", it sounds OK and it is probably negative, unless I am a supporter.

In the second one, there is no "those" or other words to indicate that we are looking at specific movies, so "some" sounds like a quantity, it just means "more than one", like "unos" or "algunos". It is actually redundant because we know it's more than one because "movies" is plural. "I've seen some movies recently" means the same. With "some", it definitely doesn't sound emphatic, though.

If in doubt, "some" *usually* means quantity / unos / algunos.