Learn English – Correct plural usage for “hundreds” and “thousands”

When you're using a countable noun in the plural, further references to that noun should honor the plural:

Dogs are the loveliest (not most lovely) animals in the world.

In the singular, you would say:

Dog is man's best friend. (example: http://mashable.com/2013/03/12/dog-mans-best-friend/)

The + countable nouns is tricky. You'd typically use it when talking about a specific instance of the countable noun, not in general. So "the dog" isn't appropriate, since we're not talking about a specific dog. There are some cases where you use "the", for example - the Moon, the Sun, the equator. All of these words have plurals, but of all moons and suns there are some of special significance to us, and so we use a definite article there.

If you're using demonstrative adjectives with a list, you should use one for each item in the list. See more on "demonstrative adjectives" on this page.

This milk, this butter, and this bread are beyond their sell-by dates.

Even though those nouns are uncountable, you use "this" and not "these," because uncountable nouns are generally treated as singular. See here for more information on that subject.

In your second example, one item is singular (uncountable and treated as singular) and one is plural, so you would use different demonstrative adjectives for each:

This evidence and these witnesses prove him guilty.

Using a demonstrative adjective is only necessary to point to which objects you're talking about. In most cases, you could avoid this problem by simply using an article like "the."

The milk, butter, and bread are beyond their sell-by dates. The evidence and witnesses prove him guilty.