"That's a lot of books."
Translation:Das sind viele Bücher.
Why is "ist" not correct here? A group as a unit can be referred to as a singular in English. Is that not allowed in German? -- That is a lot of books. Which books? That group of books. It is a group.
German does not use a collective noun here as does English ("lot"), hence you need to use the plural form of the verb (as you would in English if you said Those are many books.)
in general: viele = many, viel = much
So, one is used with countable nouns, the other with uncountable ones.
diese (explicitly plural) refers to a subset of something you had discussed before, i.e. "these ones".
The "ones" stands for whatever you had been discussing, e.g. "books".
But "These books are a lot of books" or "These ones are a lot of books" doesn't make sense.
In the English sentence, you're not referring back to something you had talked about before, but are introducing something new to the conversation -- pointing at something and saying, "That's a lot of books."
Such new topics in German are introduced in the neuter singular, e.g. with das or dies, regardless of the gender of the new thing(s) or how many there are.
In English the translation is That is a lot of books. Whilst in German the plural refers to the books separately as many: in English it refers to the books collectively as one group of books.
Could I also say "diese Bücher sind zu viele" as in: "These books are too many" ?
It sounds a bit odd to me, using zu viele predicatively like that.
I'd probably say Das sind zu viele Bücher "That's too many books; Those are too many books".
How to differentiate between 'That is a lot of books' and 'These are a lot of books'?