Correct plural usage for "hundreds" and "thousands"

I'm having problems with hundred and thousand.

If someone asks you, How many trees are in this forest? Would you answer seven thousand or seven thousands, because thousands are countable?

I know that you usually say seven thousand, but I also know that thousands are countable, so wouldn't that make it seven thousands?

And when you are talking about multiple countable nouns, you would make it plural, but the plural isn't thousand, it's thousands, so it couldn't be that thousand is the plural.

Why isn't it a few thousands? Is it because of an exception in English? Any help would be appreciated.

Answer

It is because you are not counting the thousands, you are counting the trees.
So when you say:

There are seven thousand trees.

The noun trees is countable and plural. thousand is an adjective in this example. Even if you don't say "trees" its still implied.

But you could say:

There are thousands and thousands of trees."

where "thousand" is used as a noun and you are counting groups of 1,000.