Learn French: Ce sont des monuments nationaux. - These are national monuments.

In French, masculine singular adjectives ending in -al usually change to -aux in the plural. The singular is national, so the plural becomes nationaux to match the plural masculine noun monuments.

Why do we use Ce sont instead of Ils sont to mean "These are" or "They are"?

You must use c'est or its plural ce sont when the verb is followed by a modified noun (a noun with an article, like des monuments). You would only use Ils sont here if it were followed by an adjective alone, as in Ils sont nationaux (They are national).

Why does nationaux come after monuments instead of before it?

Most adjectives in French go after the noun they describe. Only a small group of common adjectives (usually describing Beauty, Age, Number, Goodness, and Size) go before the noun. Since nationaux doesn't fit those categories, it follows the standard rule and goes after.